ARM: S3C64XX: fix uart clock setup for mini6410/real6410

ARM: S3C64XX: fix uart clock setup for mini6410/real6410

Don't rewrite clock config in UCON preconfigured by
bootloader. No need to set 10th bit in UCON because
[11:10] 2'b00 means source clock is PCLK too.
If set, console does not work if bootloader
has preconfigured [11:10] with 2'b00.
If not set, console works with any bootloader
config value (2'bxx).
More information about clock setup in UCON is available
in "S3C6410X RISC Microprocessor User's Manual,
Revision 1.20" p. 31-13 (Chapter 31.6.2
UART CONTROL REGISTER).

Signed-off-by: Darius Augulis <augulis.darius@gmail.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>