amd/pi: Add AgesaHeapRebase callout

Implement an optional callout for AgesaHeapRebase which allows AGESA
to override any internal hardcoded heap addresses.

Designate a region in CAR that may be used for pre-mem heap and return
that address before DRAM is configured.  After DRAM is up, the address
in cbmem is returned.

TEST=Boot grunt with patchstack and experimental blob
BUG=b:74518368

Change-Id: Ieda202a6064302b21707bd7ddfabc132cd85ed45
Signed-off-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Reviewed-on: https://review.coreboot.org/25458
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>
4 files changed