Kconfig: make NOR_BOOT a common option

Kconfig: make NOR_BOOT a common option

Not only am335x supports booting from NOR, i.MX6 SoCs also
supports booting from NOR. Make NOR_BOOT a common
option to let different SoCs share it.

Signed-off-by: Peng Fan <[email protected]>
Cc: Simon Glass <[email protected]>
Cc: Heiko Schocher <[email protected]>
Cc: Joe Hershberger <[email protected]>
Cc: Christophe Ricard <[email protected]>
Cc: Bin Meng <[email protected]>
Cc: Francois Retief <[email protected]>
Cc: Tom Rini <[email protected]>
Reviewed-by: Simon Glass <[email protected]>