mmc: fsl_esdhc_spl: pre-PBL: check for BOOT signature instead of MBR/DBR

mmc: fsl_esdhc_spl: pre-PBL: check for BOOT signature instead of MBR/DBR

Pre-PBL BootROMs (MPC8536E, MPC8569E, P2020, P1011, P1012, P1013, P1020,
P1021, P1022) require custom BOOT signature on sector 0 and MBR/DBR
signature is not required at all.

So add check for BOOT signature and remove check for MBR/DBR.

This allows U-Boot SPL to load proper U-Boot on pre-PBL BootROMs platforms
also from SD cards which do not have MBR/DBR signature on sector 0.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>