spi: meson: Select REGMAP_MMIO

The Meson SPIFC driver uses regmap mmio functions and so it must
select REGMAP_MMIO to avoid the following build error:

spi-meson-spifc.c: undefined reference to `devm_regmap_init_mmio_clk'

Reported-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>