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/pfla02_defconfig b/configs/pfla02_defconfig
index 6787439..a45322d 100644
--- a/configs/pfla02_defconfig
+++ b/configs/pfla02_defconfig
@@ -26,6 +26,7 @@
 CONFIG_SPL_FS_EXT4=y
 CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_SPL_SPI_LOAD=y
+CONFIG_SYS_SPI_U_BOOT_OFFS=0x10000
 CONFIG_SPL_WATCHDOG_SUPPORT=y
 CONFIG_SPL_YMODEM_SUPPORT=y
 # CONFIG_CMD_FLASH is not set