ath79: enable Regulator support for target

ath79: enable Regulator support for target

Regulator support was enabled on all subtargets except for ath79-nand.
With Kernel 5.10, AT803x requires Regulator support, thus enabling on
the complete target, as ath79-nand requires AT803x.

While this is only required on Kernel 5.10, enable it also on 5.4. We
have no major size-constraint, so enabling it on 5.4 allows us to clean
up the occurences in the subtarget configuration.

Signed-off-by: David Bauer <mail@david-bauer.net>