ARM: S5P: Remove fixed uart offset dependent code

The uart offset between channels has been 0x400 for all S5P series
and to present virtual and physical offsets, both number 0x400 and
S3C_UART_OFFSET have been used together.
This patch removes the use of the number 0x400 and expands the virtual
mapping size of S3C_VA_UART from 4KB to 512KB to support the CPU with
S3C_UART_OFFSET being 0x10000.

Signed-off-by: Changhwan Youn <chaos.youn@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>