phy: renesas: rcar-gen3-usb2: add regulator dependency

phy: renesas: rcar-gen3-usb2: add regulator dependency

The driver start registering a regulator, but can still be
enabled even when it is unable to call into the regulator
subsystem:

aarch64-linux-ld: drivers/phy/renesas/phy-rcar-gen3-usb2.o: in function `rcar_gen3_phy_usb2_probe':
phy-rcar-gen3-usb2.c:(.text+0x2884): undefined reference to `devm_regulator_register'

Add a Kconfig dependency to avoid this configuration.

Fixes: b6d7dd157763 ("phy: renesas: rcar-gen3-usb2: Add regulator for OTG VBUS control")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Tommaso Merciai <tommaso.merciai.xr@bp.renesas.com>
Link: https://patch.msgid.link/20260202095118.1233046-1-arnd@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>