powerpc: mpc85xx: Fix loading U-Boot proper from SD card in SPL

Change 8-byte alignment of SPL binary to just 4-byte alignment as objcopy
trims trailing zero bytes when converting ELF file to RAW binary.

This is same fix for SPL linker script as was done fix for U-Boot linker
script in commit e8c0e006 ("powerpc: mpc85xx: Fix CONFIG_OF_SEPARATE
support").

It is required for the patch "mmc: fsl_esdhc_spl: Add support for builds
without CONFIG_SYS_MMC_U_BOOT_OFFS" which triggered this issue in SPL.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <kabel@kernel.org>