pinctrl: pic64gx-gpio2: Add REGMAP_MMIO dependency

pinctrl: pic64gx-gpio2: Add REGMAP_MMIO dependency

In line with other drivers depending on REGMAP_*, select the required
symbol to prevent a linker error when building with COMPILE_TEST=y:

ld: drivers/pinctrl/pinctrl-pic64gx-gpio2.o: in function `pic64gx_gpio2_probe':
pinctrl-pic64gx-gpio2.c:315:(.text+0x198): undefined reference to `__devm_regmap_init_mmio_clk'

Fixes: 38cf9d641314 ("pinctrl: add pic64gx "gpio2" pinmux driver")
Signed-off-by: Sander Vanheule <sander@svanheule.net>
Signed-off-by: Linus Walleij <linusw@kernel.org>