Not all targets need regulator support, so they dont enable it as its
disabled in the generic config by default.
So, in order to allow kernel modules to depend on regulator support lets
add a new feature flag "regulator" and set it automatically if target
kernel config enables CONFIG_REGULATOR.
Link: https://github.com/openwrt/openwrt/pull/22172
Signed-off-by: Robert Marko <robimarko@gmail.com>