_DSM: OS Request to Modify FW-Owned Register

This Change defines a new Device-Specific Function. This Function allows OS software to request
the change of bits within a PCI register that is under ownership of Platform Firmware.

This new DSM takes, as input, a value denoting the register to be modified, a mask value to select
the bits within the register to be modified, and a new value for those bits.

This DSM returns values indicating success, or error codes such as invalid input values.