usb: dwc3: select CONFIG_REGMAP_MMIO

After many randconfig builds, one configuration caused a link
error with dwc3-meson-g12a lacking the regmap-mmio code:

drivers/usb/dwc3/dwc3-meson-g12a.o: In function `dwc3_meson_g12a_probe':
dwc3-meson-g12a.c:(.text+0x9f): undefined reference to `__devm_regmap_init_mmio_clk'

Add the select statement that we have for all other users
of that dependency.

Fixes: c9999337

 ("usb: dwc3: Add Amlogic G12A DWC3 glue")
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>