cmd: fastboot: Enable FASTBOOT_FLASH_NAND for SUNXI_NAND devices

Encountered an issue where fastboot can't write to NAND on a CHIP_pro,
the symbol was neither present in the board's config header, nor the
Kconfig, this patch puts it in the Kconfig and defaults on when
SUNXI_NAND is selected.

Signed-off-by: Ben Young <computermouth@crunchbangplusplus.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>