spl: nand: rework SYS_NAND_U_BOOT_OFFS Kconfig option dependency

spl: nand: rework SYS_NAND_U_BOOT_OFFS Kconfig option dependency

The SYS_NAND_U_BOOT_OFFS is quite generic, but the Kconfig entry is forced
to explicitly depend on platforms that are not already defining it in their
include/configs/<board>.h header.

Add the SYS_NAND_U_BOOT_LOCATIONS option, make the SYS_NAND_U_BOOT_OFFS
depends on it, remove the dependency on NAND_SUNXI and make it dependent
on SPL selection.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>