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/display5_defconfig b/configs/display5_defconfig
index 13db0e8..8609cd5 100644
--- a/configs/display5_defconfig
+++ b/configs/display5_defconfig
@@ -32,6 +32,7 @@
 CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_SPL_OS_BOOT=y
 CONFIG_SPL_SPI_LOAD=y
+CONFIG_SYS_SPI_U_BOOT_OFFS=0x20000
 CONFIG_SPL_WATCHDOG_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="display5 > "