xilinx: zynqmp: Config non zero SYS_SPI_U_BOOT_OFFS

This variable is pointing to offset is qspi where u-boot image is placed.
In our case it is location of u-boot.itb file. Offset is the same as is
used by Xilinx Zynq SoC.
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>

configs/xilinx_zynqmp_virt_defconfig

@@ -5,6 +5,7 @@ CONFIG_SYS_TEXT_BASE=0x8000000
 CONFIG_SYS_MALLOC_F_LEN=0x8000
 CONFIG_SYS_MEMTEST_START=0x00000000
 CONFIG_SYS_MEMTEST_END=0x00001000
+CONFIG_SYS_SPI_U_BOOT_OFFS=0x100000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="zynqmp-zcu100-revC"
 CONFIG_SPL=y