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/socfpga_is1_defconfig b/configs/socfpga_is1_defconfig
index f369301..6ea06c1 100644
--- a/configs/socfpga_is1_defconfig
+++ b/configs/socfpga_is1_defconfig
@@ -15,6 +15,7 @@
 CONFIG_DISPLAY_BOARDINFO_LATE=y
 CONFIG_SPL_TEXT_BASE=0xFFFF0000
 CONFIG_SPL_SPI_LOAD=y
+CONFIG_SYS_SPI_U_BOOT_OFFS=0x40000
 CONFIG_CMD_ASKENV=y
 CONFIG_CMD_GREPENV=y
 # CONFIG_CMD_FLASH is not set