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 <peng.fan@nxp.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Heiko Schocher <hs@denx.de>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Christophe Ricard <christophe-h.ricard@st.com>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Francois Retief <fgretief@spaceteq.co.za>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>