misc: isa1200: Add mutex for clk_on

clk_on is a boolean which maintains the enable/disbale
status of clock. To avoid the race condition, mutex call
is added for the clk_on.

CRs-fixed: 507663
Change-Id: If6d933568abc747f3b18497b2c2e743fff4c9e04
Signed-off-by: Shantanu Jain <shjain@codeaurora.org>