drm/amdgpu: add powerplay sclk OD support through sysfs (v2)

Add a new sysfs entry pp_sclk_od to support sclk overdrive(OD) overclocking,
the entry is read/write, the value of input/output is an integer which is the
over percentage of the highest sclk.

v2: drop extra semicolon

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Eric Huang <JinHuiEric.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>