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/dra7xx_hs_evm_usb_defconfig b/configs/dra7xx_hs_evm_usb_defconfig
index 6d6bfbc..040e6f7 100644
--- a/configs/dra7xx_hs_evm_usb_defconfig
+++ b/configs/dra7xx_hs_evm_usb_defconfig
@@ -34,6 +34,7 @@
 # CONFIG_SPL_NAND_SUPPORT is not set
 CONFIG_SPL_RAM_SUPPORT=y
 CONFIG_SPL_SPI_LOAD=y
+CONFIG_SYS_SPI_U_BOOT_OFFS=0x40000
 CONFIG_SPL_USB_GADGET=y
 CONFIG_SPL_DFU=y
 CONFIG_SPL_YMODEM_SUPPORT=y