clk: imx51: uart4, uart5 gates only exist on imx50, imx53

clk: imx51: uart4, uart5 gates only exist on imx50, imx53

i.MX51 only has 3 UARTs and no CCGR7 register. In place of the CCGR7
register on i.MX50/i.MX53 that contains the ipg and per clock gates
for UARTs 4 and 5, on i.MX51 there is the CMEOR register.

Without this patch, the code disabling the UART clocks would also clear
the mod_en_ov_vpu bit in the CMEOR register, among others, which causes
register accesses to the VPU to lock up the system.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>