net: pse-pd: PSE_REGULATOR should depend on REGULATOR

The Regulator based PSE controller driver relies on regulator support to
be enabled.  If regulator support is disabled, it will still compile
fine, but won't operate correctly.

Hence add a dependency on REGULATOR, to prevent asking the user about
this driver when configuring a kernel without regulator support.

Fixes: 66741b4e

 ("net: pse-pd: add regulator based PSE driver")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/709caac8873ff2a8b72b92091429be7c1a939959.1664900558.git.geert+renesas@glider.be
Signed-off-by: Jakub Kicinski <kuba@kernel.org>