arm64: zynqmp: Add support for generic QSPI boot

This patch is enabling support for SPL QSPI boot.

First of all it is necessary to generate atf-spi.ub which is different
format than atf-uboot.ub (this can be made as legacy image too)

ADDR=`arm-xilinx-linux-gnueabi-readelf -a bl31.elf | grep "Entry point
address" | cut -d ':' -f 2 | sed -e 's/^[ \t]*//'`
aarch64-linux-gnu-objcopy -O binary bl31.elf bl31.bin
./tools/mkimage -f auto -A arm64 -T firmware -C none -O u-boot -a $ADDR
-e $ADDR -n "atf1" -E -b arch/arm/dts/zynqmp-zcu102.dtb -d bl31.bin
atf-uboot.ub
./tools/mkimage -A arm64 -T firmware -C none -O u-boot -a $ADDR -e $ADDR
-n "atf-for-qspi" -E -d bl31.bin atf-spi.ub

This patch is using this QSPI layout with offsets:
0 boot.bin
512k atf-ub
640k u-boot.bin
1280k u-boot.img

Which corresponding by writing these images(read from MMC)
mmcinfo
sf probe
load mmc 0 10000000 boot.bin
sf erase 0 +$filesize
sf write 10000000 0 $filesize
load mmc 0 10000000 atf-spi.ub
sf erase 0x80000 +$filesize
sf write 10000000 0x80000 $filesize
load mmc 0 10000000 u-boot.bin
sf erase 0xa0000 +$filesize
sf write 10000000 0xa0000 $filesize
load mmc 0 10000000 u-boot.img
sf erase 0x140000 +$filesize
sf write 10000000 0x140000 $filesize

For testing u-boot running in EL3 you can break atf-spi.ub like this:
sf probe
sf erase 0x80000 +4

Then u-boot.img is executed.

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

arch/arm/dts/zynqmp.dtsi

@@ -804,6 +804,7 @@
 		};
 
 		qspi: spi@ff0f0000 {
+			u-boot,dm-pre-reloc;
 			compatible = "xlnx,zynqmp-qspi-1.0";
 			status = "disabled";
 			clock-names = "ref_clk", "pclk";

include/configs/xilinx_zynqmp.h

@@ -234,6 +234,15 @@
 
 #define CONFIG_SPL_FRAMEWORK
 
+#if defined(CONFIG_SPL_SPI_FLASH_SUPPORT)
+# define CONFIG_SPL_SPI_LOAD
+# define CONFIG_SYS_SPI_KERNEL_OFFS	0x80000
+# define CONFIG_SYS_SPI_ARGS_OFFS	0xa0000
+# define CONFIG_SYS_SPI_ARGS_SIZE	0xa0000
+
+# define CONFIG_SYS_SPI_U_BOOT_OFFS	0x170000
+#endif
+
 /* u-boot is like dtb */
 #define CONFIG_SPL_FS_LOAD_ARGS_NAME	"u-boot.bin"
 #define CONFIG_SYS_SPL_ARGS_ADDR	0x8000000