soc/amd/stoneyridge: Add XHCI PM register access functions

Add functions to access the XHCI PM MMIO registers.

Change-Id: I81b4c0a448eb17c5ee0562a2c3548a074d533a98
Signed-off-by: Marc Jones <marcj303@gmail.com>
Reviewed-on: https://review.coreboot.org/22677
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>
3 files changed