pmbus: Add mp2971/mp2973 support in mp2975

Add mp2971/mp2973 support in mp2975

Tested with:
My unit only include mp2971 and mp2973 devices
MP2973:
cat /sys/bus/i2c/devices/5-0076/hwmon/hwmon24/*label
iin
iout1
iout2
vin
vout1
vout2
pin
pout1
pout2
cat /sys/bus/i2c/devices/5-0076/hwmon/hwmon24/*input
0
82500
14000
12187
1787
1793
0
148000000
25000000
54000
MP2971:
cat /sys/bus/i2c/devices/5-0062/hwmon/hwmon20/*label
iin
iout1
iout2
vin
vout1
vout2
pin
pout1
pout2
cat /sys/bus/i2c/devices/5-0062/hwmon/hwmon20/*input
18500
22000
500
12187
1025
1800
226000000
22000000
1000000
55000

Signed-off-by: Wang Xiaohua <wangxiaohua.1217@bytedance.com>