clk: mpfs_clk: Enable DM_FLAG_PRE_RELOC flag

This driver is needed in the pre-relocation phase as the serial
driver depends on it.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Padmarao Begari <padmarao.begari@microchip.com>
Tested-by: Padmarao Begari <padmarao.begari@microchip.com>