mfd: rk8xx: Add rk806 support

Add support for SPI connected rk806, which is used by the RK3588
evaluation boards. The PMIC is advertised to support I2C and SPI,
but the evaluation boards all use SPI. Thus only SPI support is
added here.

Tested-by: Diederik de Haas <didi.debian@cknow.org> # Rock64, Quartz64 Model A + B
Tested-by: Vincent Legoll <vincent.legoll@gmail.com> # Pine64 QuartzPro64
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/20230504173618.142075-9-sebastian.reichel@collabora.com


Signed-off-by: Lee Jones <lee@kernel.org>