nandbcb: nand support for i.MX8MP

Add NAND boot support for i.MX8MP by adding i.MX8MP in nandbcb support
list, reading boot_search_count from fuse.

i.MX8MN NAND boot is same as i.MX8MP, fix some issues as well.

Signed-off-by: Han Xu <han.xu@nxp.com>
Reviewed-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>