Support RAM and MMC boot mode in SPL also with SPL_FIT images.

In MMC boot mode two boot options are available:
1) Boot flow with ATF(EL3) and full U-Boot(EL2):
 aarch64-linux-gnu-objcopy -O binary bl31.elf bl31.bin
 mkimage -A arm64 -O linux -T kernel -C none -a 0xfffe5000 -e 0xfffe5000
 -d bl31.bin atf.ub
 cp spl/boot.bin <sdcard fat partition>
 cp atf.ub <sdcard fat partition>
 cp u-boot.bin <sdcard fat partition>

2) Boot flow with full U-Boot(EL3):
 cp spl/boot.bin <sdcard>
 cp u-boot*.img <sdcard>

3) emmc boot mode
 dd if=/dev/zero of=sd.img bs=1024 count=1024
 parted sd.img mktable msdos
 parted sd.img mkpart p fat32 0% 100%
 kpartx -a sd.img
 mkfs.vfat /dev/mapper/loop0p1
 mount /dev/mapper/loop0p1 /mnt/
 cp spl/boot.bin /mnt
 cp u-boot.img /mnt
 cp u-boot.bin /mnt
 cp atf.ub /mnt
 umount /dev/mapper/loop0p1
 kpartx -d sd.img
 cp sd.img /tftpboot/

 and program it via u-boot
 tftpb 10000 sd.img
 mmcinfo
 mmc write 10000 0 $filesize
 mmc rescan
 mmc part
 ls mmc 0

psu_init() function contains low level SoC setup generated for every HW
design by Xilinx design tools. xil_io.h is only supporting file to fix
all dependencies from tools. The same solution was used on Xilinx Zynq.

The patch also change CONFIG_SYS_INIT_SP_ADDR to the end of OCM which
stays at the same location all the time.
Bootrom expects starting address to be at 0xfffc0000 that's why this
address is SPL_TEXT_BASE.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>