sunxi: move CONFIG_SYS_SPI_U_BOOT_OFFS out of defconfig

sunxi: move CONFIG_SYS_SPI_U_BOOT_OFFS out of defconfig

For Allwinner SoCs the CONFIG_SYS_SPI_U_BOOT_OFFS value is not really a
board choice: The boot ROM only loads the SPL from offset 0 of the SPI
NOR flash, and loads at most 32KB. This is a similar situation as on MMC,
so consequently we create our "joint" image (SPL + U-Boot proper) with
that 32KB offset during the build.

So define the value of this symbol to be 32KB by default for every
Allwinner SoC. This removes the definition of this symbol from the
_defconfig files, and avoids every board to define this over and over
again.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>