follow up on #7241

Since #7241 was merged, there is no need anymore for ugly file guards around peripheral driver implementations, as they are now finally only touched by the compiler if they are actually (a) included in the build (using USEMODULE) and (b) provided by a board/cpu (as in FEATURES_PROVIDED). So now it is time to clean up a little bit, and this issue should help so we don't forget anything.

Removed file guards from peripheral drivers in:

• arm7_common
• atmega1281
• atmega2560
• atmega328p
• atmega_common (cpu/atmega_common: remove useless periph file guard #8959)
• cc2538 (cpu/cc2538: remove obsolete periph file guard #8955)
• cc26x0
• cc430
• cortexm_common
• efm32
• ezr32wg
• k22f
• k60
• k64f
• kinetis_common (-> @gebart, kinetis: Support all models in one directory #7882)
• kw2xd
• lm4f120 (cpu/lm4f120: remove useless periph file guard #8958)
• lpc1768 (cpu/lpc1768: remove useless timer periph file guard #8956) (-> @aabadie, cpu/lpc1768: remove useless timer periph file guard #8956)
• lpc2387 (cpu/lpc2387: remove useless periph file guard #8957)
• mips32r2_common
• mips_pic32_common
• mips_pic32mx
• mips_pic32mz
• msp430_common
• msp430fxyz
• native
• nrf51 (-> @haukepetersen, cpu/nrf51: remove guards from periph drivers #7982)
• nrf52 (-> @haukepetersen, cpu/nrf51: remove guards from periph drivers #7982)
• nrf5x_common (-> @haukepetersen, cpu/nrf51: remove guards from periph drivers #7982)
• sam0_common ( -> @haukepetersen, cpu/samX: remove guards from periph drivers #7984)
• sam3 ( -> @haukepetersen, cpu/samX: remove guards from periph drivers #7984)
• sam_common
• samd21 ( -> @haukepetersen, cpu/samX: remove guards from periph drivers #7984)
• saml21 ( -> @haukepetersen, cpu/samX: remove guards from periph drivers #7984)
• stellaris_common
• stm32_common (-> @haukepetersen, cpu/stm32: removed file guards from periph drivers #7983)
• stm32f0 (-> @haukepetersen, cpu/stm32: removed file guards from periph drivers #7983)
• stm32f1 (-> @haukepetersen, cpu/stm32: removed file guards from periph drivers #7983)
• stm32f2 (-> @haukepetersen, cpu/stm32: removed file guards from periph drivers #7983)
• stm32f3 (-> @haukepetersen, cpu/stm32: removed file guards from periph drivers #7983)
• stm32f4 (-> @haukepetersen, cpu/stm32: removed file guards from periph drivers #7983)
• stm32f7 (-> @haukepetersen, cpu/stm32: removed file guards from periph drivers #7983)
• stm32l0 (-> @haukepetersen, cpu/stm32: removed file guards from periph drivers #7983)
• stm32l1 (-> @haukepetersen, cpu/stm32: removed file guards from periph drivers #7983)
• stm32l4 (-> @haukepetersen, cpu/stm32: removed file guards from periph drivers #7983)
• x86