mtd: nand: Make CONFIG_SYS_NAND_U_BOOT_OFFS configurable through Kconfig

mtd: nand: Make CONFIG_SYS_NAND_U_BOOT_OFFS configurable through Kconfig

Make CONFIG_SYS_NAND_U_BOOT_OFFS configurable through Kconfig, just like
SYS_NAND_BUSWIDTH_16BIT this is only enabled on some SoCs using depends,
to avoid double defining it for SoCs which have not yet moved to Kconfig
for this.

Having this in Kconfig is useful because this is something which may
differ from one board to the other even when using the same SoC.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>
Acked-by: Scott Wood <scottwood@freescale.com>