MMC: OMAP: Lazy clock shutdown

MMCA spec says the mmc clock should be kept running for at least
8 cycles after the last RW request. Ensure this with lazy clock
disable after a request, or with an explicit delay before
switching a slot.

Signed-off-by: Jarkko Lavinen <jarkko.lavinen@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>