driver: pwm: give arc the access to pwm

HW allows both arc and x86 to access the pwm attached to the
quark soc I/O fabric. The existing code only allows x86 to
do this. Let's give arc the access.

Change-Id: Ie88649fea41bf86b84876c260f97dd4c12fd0b81
Signed-off-by: Baohong liu <baohong.liu@intel.com>