hwmon: (pmbus) add a function to check the presence of a block register

Other functions (like pmbus_check_byte_register) cannot be used to check
the presence of a block register, because it will generate error when PEC
is used.

Signed-off-by: Adam Wujek <dev_public@wujek.eu>
Link: https://lore.kernel.org/r/20220601013232.801133-1-dev_public@wujek.eu


Signed-off-by: Guenter Roeck <linux@roeck-us.net>