pwm: Add BCM2835 SoC PWM driver

Add support for the PWM controller of the BCM2835 SoC found on the Raspberry PI

Signed-off-by: Johannes Thumshirn <morbidrsa@gmail.com>

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# On branch rpi-pwm
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#	modified:   drivers/pwm/Kconfig
#	modified:   drivers/pwm/Makefile
#	new file:   drivers/pwm/pwm-bcm2835.c
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