pwm: Add Raspberry Pi Firmware based PWM bus

Adds support to control the PWM bus available in official Raspberry Pi
PoE HAT. Only RPi's co-processor has access to it, so commands have to
be sent through RPi's firmware mailbox interface.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>

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Changes since v2:
 - Use devm_rpi_firmware_get()
 - Rename driver
 - Small cleanups

Changes since v1:
 - Use default pwm bindings and get rid of xlate() function
 - Correct spelling errors
 - Correct apply() function
 - Round values
 - Fix divisions in arm32 mode
 - Small cleanups
3 files changed