regulator: add support for SY8106A regulator

SY8106A is an I2C attached single output regulator made by Silergy Corp,
which is used on several Allwinner H3/H5 SBCs to control the power
supply of the ARM cores.

Add a driver for it.

Signed-off-by: Ondrej Jirman <megous@megous.com>
[Icenowy: Change commit message, remove enable/disable code, add default
 ramp_delay, add comment for go bit, add code for fixed mode voltage]
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Mark Brown <broonie@kernel.org>