ARM: imx: add mmdc ipg clock operation for mmdc

[ Upstream commit 9454a0caff6ac6d2a5ea17dd624dc13387bbfcd3 ]

i.MX6 SoCs have MMDC ipg clock for registers access, to make
sure MMDC registers access successfully, add optional clock
enable for MMDC driver.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>