hwmon: (pmbus) add helpers for byte write and read modify write

Add two helper functions:
 * pmbus_write_byte_data  = paged byte write
 * pmbus_update_byte_data = paged byte read/modify/write

Signed-off-by: Alan Tull <atull@opensource.altera.com>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>