mfd: davinci: Voicecodec needs regmap_mmio

Without REGMAP_MMIO, building that driver results in a link error:

drivers/built-in.o: In function `davinci_vc_probe':
:(.init.text+0x3c1c): undefined reference to `devm_regmap_init_mmio_clk'

This adds a Kconfig 'select' statement as the usual way to ensure
that REGMAP_MMIO is enabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>