ARM: SAMSUNG: Add a workaround for get_clock() for serial driver

Serial drivers call get_clock() very early, before platform bus
has been set up, this requires a special check to let them get
a proper clock. Without this patch, a serial console is broken
on S5PV310 and S5PC210 boards.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
[kgene.kim@samsung.com: fix coding-style]
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>
Cc: Ben Dooks <ben-linux@fluff.org>