arm/imx: use Kconfig choice for low-level debug UART selection

Now that the DEBUG_LL UART can be selected by a Kconfig choice,
simplify the #ifdefery in debug-macro.S and add entries to the
top-level Kconfig.debug instead.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Will Deacon <will.deacon@arm.com>