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/orangepi_zero_defconfig b/configs/orangepi_zero_defconfig
index a7c72f7..f8dda05 100644
--- a/configs/orangepi_zero_defconfig
+++ b/configs/orangepi_zero_defconfig
@@ -9,6 +9,7 @@
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
 CONFIG_USE_PREBOOT=y
 CONFIG_CONSOLE_MUX=y
+CONFIG_SYS_SPI_U_BOOT_OFFS=0x8000
 # CONFIG_CMD_FLASH is not set
 CONFIG_DEFAULT_DEVICE_TREE="sun8i-h2-plus-orangepi-zero"
 CONFIG_SUN8I_EMAC=y