ARM: imx: support DEBUG_LL uart port selection for all i.MX SoCs

Extend imx6q DEBUG_LL uart port selection support to cover all i.MX
SoCs.  The 'range' of the Kconfig option gets dropped, as users
looking at the option must know the uart number on his board.  The
bottom line is that the build system will report an error if an
invalid port number is picked for given SoC.

The header arch/arm/include/debug/imx-uart.h is created to accommodate
all the uart base addresses.  And the header will also be used for
other low-level debug facility later.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>