imx8m: Refactor the OPTEE memory removal

Current codes assume the OPTEE address is at the end of first DRAM bank.
Adjust the process to allow OPTEE in the middle of first bank.

When OPTEE memory is removed from first bank, it may split the first bank
to two banks, adjust the MMU table for the split case,
Since the default CONFIG_NR_DRAM_BANKS is 4, it is enough, just enlarge
i.MX8MP evk to default to avoid issue.

Signed-off-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Silvano di Ninno <silvano.dininno@nxp.com>
Tested-by: Silvano di Ninno <silvano.dininno@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>