clock-plutonium: Rename SVS2 to LOWER

SVS2 is a name used by hardware team. Use LOWER instead to be consistent
with previous naming scheme in clock driver. Also update comments about
SVS2.

Change-Id: If6b395326c7845b903756cefede677ae3a2b05e0
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>