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: 629529
Change-Id: I4f61c629c503c658410b5c14da2e1612b82ec201
Signed-off-by: Sarada Prasanna Garnayak <c_sgarna@codeaurora.org>
Signed-off-by: revjunkie <gadgetjuan@gmail.com>