EXYNOS5: Enable console multiplexing in u-boot

EXYNOS5: Enable console multiplexing in u-boot

We enable console multiplexing and use both serial and LCD for stdout/stderr.
Initially, u-boot output console is observed via serial port.
If you also have a DP panel connected onto your SMDK5250 board,
you can switch to LCD console by typing "setenv stdout lcd".
You can always switch back to serial using "setenv stdout serial".
You can switch error console(stderr) as well, using similar commands.

Signed-off-by: Ajay Kumar <ajaykumar.rs@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>