armv8: fsl-layerscape: Increase mmc read size for secure-boot headers

Maximum size of secure boot header to be read from MMC is
12KB which spans across 0x20 blocks.

Hence increase the mmc read size for secure boot
headers from MMC to 0x20 blocks.

Signed-off-by: Udit Agarwal <udit.agarwal@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>