mmc: fsl_esdhc_spl: Add support for builds without CONFIG_SYS_MMC_U_BOOT_OFFS

When fixed offset via CONFIG_SYS_MMC_U_BOOT_OFFS is not specified then
expects that U-Boot proper is placed immediately after SPL without any
additional padding.

This allows to generate smaller SPL+U-Boot final binary as it is not
required to specify fixed offset to U-Boot proper at SPL compile time.

In this case offset to U-Boot proper is calculated at SPL compile time in
linker script.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Marek Behún <kabel@kernel.org>