board: freescale: p1_p2_rdb_pc: Simplify SPL offset macros

Now when CONFIG_SYS_TEXT_BASE has sane value, use it for calculation of
other SPL offset values: CONFIG_SPL_MAX_SIZE, CONFIG_SYS_MMC_U_BOOT_* and
CONFIG_SYS_SPI_FLASH_U_BOOT_* macros.

No functional change.

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