cpu: efm32_common: add peripheral drivers

cpu/efm32_common/Makefile

cpu/efm32_common/Makefile.dep

cpu/efm32_common/Makefile.features

cpu/efm32_common/Makefile.include

cpu/efm32_common/cpu.c

cpu/efm32_common/doc.txt

cpu/efm32_common/include/periph_cpu.h

cpu/efm32_common/periph/Makefile

cpu/efm32_common/periph/adc.c

cpu/efm32_common/periph/cpuid.c

cpu/efm32_common/periph/dac.c

cpu/efm32_common/periph/flashpage.c

cpu/efm32_common/periph/gpio.c

cpu/efm32_common/periph/hwrng.c

cpu/efm32_common/periph/i2c.c

cpu/efm32_common/periph/pm.c

cpu/efm32_common/periph/pwm.c

cpu/efm32_common/periph/rtc_series0.c

cpu/efm32_common/periph/rtc_series1.c

cpu/efm32_common/periph/rtt_series0.c