soc: rockchip: grf: switch to FIELD_PREP_WM16_CONST macro

The era of hand-rolled HIWORD_UPDATE macros is over, at least for those
drivers that use constant masks.

Switch the rockchip grf driver to the FIELD_PREP_WM16_CONST macro, which
brings with it more error checking while still being able to be used in
initializers.

All HIWORD_UPDATE instances and its definition are removed from the
driver, as the conversion here is obvious, and static_asserts were used
during development to make sure the ones greater than one bit in width
were really equivalent.

Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>