Use the pp_od_clk_voltage sysfs file to configure the min and max value
of gfx clock frequency or to restore the default values.

Command guide:
echo "s level value" > pp_od_clk_voltage
        "s" - set the sclk frequency
        "level" - 0 or 1, "0" represents the min value, "1" represents
        the max value
        "value" - the target value of sclk frequency, it should be
        limited in the safe range
echo "r" > pp_od_clk_voltage
        "r" - reset the sclk frequency, restore the default value
echo "c" > pp_od_clk_voltage
        "c" - commit the min and max value of sclk frequency to the
        system only after the commit command, the setting target values
        by "s" command will take effect

Example:
1) check the default sclk frequency
        $ cat pp_od_clk_voltage
        OD_SCLK:
        0:        200Mhz
        1:        600Mhz
        OD_RANGE:
        SCLK:     200MHz       600MHz
2) use "s" -- set command to configure the min or max sclk frequency
        $ echo "s 0 300" > pp_od_clk_voltage
        $ echo "s 1 500" > pp_od_clk_voltage
        $ echo "c" > pp_od_clk_voltage
        $ cat pp_od_clk_voltage
        OD_SCLK:
        0:        300Mhz
        1:        500Mhz
        OD_RANGE:
        SCLK:     200MHz       600MHz
3) use "r" -- reset command to restore the min and max sclk frequency
        $ echo "r" > pp_od_clk_voltage
        $ echo "c" > pp_od_clk_voltage
        $ cat pp_od_clk_voltage
        OD_SCLK:
        0:        200Mhz
        1:        600Mhz
        OD_RANGE:
        SCLK:     200MHz       600MHz

v2: modify the description of reset command usage - need to do "commit"
    after set the restore command

Signed-off-by: Xiaomeng Hou <Xiaomeng.Hou@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>