sysreset: add support for socfpga sysreset

This moves sysreset support for socfgpa from ad-hoc code in mach-socfpga
to a UCLASS_SYSRESET based dm driver.

A side effect is that gen5 and a10 can now select between cold and warm
reset.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>