mmc: atmel-mci: add vmmc-supply support

Other MMC hosts handle a regulator named vmmc-supply that allows to power
the MMC card or SDIO device before communicating on the bus.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Chris Ball <chris@printf.net>