ARM: S5P64X0: Add UART serial support for S5P6450

This patch adds UART serial port support for S5P6450 SoC.
The S5P6450 has 6 UARTs, so adds resource of UART4 and UART5.
And to fix membase which is in serial/samsung.c is from Ben Dooks.

Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>
Cc: Ben Dooks <ben-linux@fluff.org>