pmdomain: sunxi: sun20i-ppu: change to tristate and enable for ARCH_SUNXI

pmdomain: sunxi: sun20i-ppu: change to tristate and enable for ARCH_SUNXI

There is no reason why the sun20i-ppu cannot be built as a module. So
change it to tristate.

Also enable it by default for ARCH_SUNXI since this driver is required
for some peripherals to work, and update the help text to reflect this
requirement.

This aligns it with the new PCK-600 driver.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Link: https://lore.kernel.org/r/20250712074021.805953-5-wens@kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>