regulator: rk8xx: add indirection level for some ldo callbacks

By passing a rk8xx_reg_info directly to the internal get_value, it'd be
possible to call this same function with a logic for getting the
rk8xx_reg_info different from the current get_ldo_reg, e.g. for NLDO and
PLDO support for RK806.

No logic change is expected.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>