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_win_defconfig b/configs/orangepi_win_defconfig
index 8a9cc40..2839503 100644
--- a/configs/orangepi_win_defconfig
+++ b/configs/orangepi_win_defconfig
@@ -7,6 +7,7 @@
 CONFIG_SPL_SPI_SUNXI=y
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
 CONFIG_USE_PREBOOT=y
+CONFIG_SYS_SPI_U_BOOT_OFFS=0x8000
 # CONFIG_CMD_FLASH is not set
 # CONFIG_SPL_DOS_PARTITION is not set
 # CONFIG_SPL_EFI_PARTITION is not set