dts: Make camN_reg and camN_reg_gpio overrides generic

The camera regulator GPIO can be used for other purposes,
so the camN_reg override to allow disabling is potentially
useful on any platform.
camN_gpio is less useful, but isn't invalid.

Move these overrides from the CM dt files to bcm270x-rpi.dtsi
and bcm2712-rpi.dtsi.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>