ARM: imx: use separated debug uart symbol for imx31 and imx35

It's improper to use combined symbol DEBUG_IMX31_IMX35_UART for
imx31 and imx35, because imx31 has 5 UARTs while imx35 only has 3.
So later when we add UART port selection for DEBUG_IMX31_IMX35_UART,
we will have problem.

Use separated Kconfig options for imx31 and imx35 debug uart selection.

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