amd/stoneyridge: Convert BiosRam access to MMIO

Change the BiosRam read/write functions to use the fixed MMIO range at
0xfed80500.  This is faster than two accesses per byte when using I/O
0xcd4/0xcd5.

Note that BiosRam may only be accessed byte-by-byte.  It does not decode
normally.

Change-Id: I9d8baf2bd5d9d48a87bddfb6a0b86e292a8fdf7d
Signed-off-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Reviewed-on: https://review.coreboot.org/23436
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
2 files changed