MX6 SabreSD: Use readl() to read the CCM_CCGR3 register

Align with the context to use readl() to read the CCM_CCGR3
register with memory barrier instead of __raw_readl().

Signed-off-by: Liu Ying <Ying.Liu@freescale.com>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>