board: freescale: p1_p2_rdb_pc: Calculate offsets for eSDHC boot sector

Correctly calculate offsets between SPL and proper U-Boot when new config
option CONFIG_FSL_PREPBL_ESDHC_BOOT_SECTOR for generating eSDHC boot sector
is enabled. Otherwise SPL would not be able to boot proper U-Boot.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>