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/chromebook_speedy_defconfig b/configs/chromebook_speedy_defconfig
index 7891d62..5319ecb 100644
--- a/configs/chromebook_speedy_defconfig
+++ b/configs/chromebook_speedy_defconfig
@@ -26,6 +26,7 @@
 # CONFIG_SPL_CRC32_SUPPORT is not set
 CONFIG_SPL_PAYLOAD="u-boot.img"
 CONFIG_SPL_SPI_LOAD=y
+CONFIG_SYS_SPI_U_BOOT_OFFS=0x20000
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_GPT=y
 CONFIG_CMD_I2C=y