apm821xx: move CONFIG_REGULATOR to target config

apm821xx: move CONFIG_REGULATOR to target config

This moves CONFIG_REGULATOR to the target config instead of the
subtarget config.

For kernel 5.10, CONFIG_AT803X_PHY depends on CONFIG_REGULATOR.
As we do not have a size constraint, move this symbol to the
target configuration for kernel 5.4 as well as 5.10.

Reported-by: Russell Senior <russell@personaltelco.net>
Signed-off-by: David Bauer <mail@david-bauer.net>