mxcmmc: convert to pm_ops and enable/disable clock

Signed-off-by: Eric Bénard <eric@eukrea.com>
Cc: s.hauer@pengutronix.de
Cc: linux-mmc@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>