ARM: S3C64XX: Add support for synchronous clock operation

Some boards based on S3C6410 use synchronous clocking, which means that HCLKx2
and other system clocks are generated from APLL instead of MPLL.

This patch adds support for such boards, by calculating hclk2 depending on
the status of S3C_OTHERS_SYNCMUXSEL bit in S3C64XX_OTHERS regist

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>