ARM: imx: rename DEBUG_IMX21_IMX27_UART to DEBUG_IMX27_UART

Since commit 4b563a06 ("ARM: imx: Remove imx21 support"), the config
DEBUG_IMX21_IMX27_UART is really only debug support for IMX27.

So, rename this option to DEBUG_IMX27_UART and adjust dependencies in
Kconfig and rename the definitions to IMX27 as further clean-up.

This issue was discovered with ./scripts/checkkconfigsymbols.py, which
reported that DEBUG_IMX21_IMX27_UART depends on the non-existing config
SOC_IMX21.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>