Convert CONFIG_SYS_SPI_U_BOOT_OFFS to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_SPI_U_BOOT_OFFS

Signed-off-by: Hannes Schmelzer <hannes.schmelzer@br-automation.com>
[trini: Expose this for SPL_SPI_SUNXI for now]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/zynq_zc770_xm010_defconfig b/configs/zynq_zc770_xm010_defconfig
index 6458e62..53108ff 100644
--- a/configs/zynq_zc770_xm010_defconfig
+++ b/configs/zynq_zc770_xm010_defconfig
@@ -19,6 +19,7 @@
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_OS_BOOT=y
 CONFIG_SPL_SPI_LOAD=y
+CONFIG_SYS_SPI_U_BOOT_OFFS=0x100000
 CONFIG_SYS_PROMPT="Zynq> "
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_FPGA_LOADBP=y