soc/amd/picasso: Use cbfs to locate the AMD firmware

soc/amd/picasso: Use cbfs to locate the AMD firmware

Switch from locating the AMD firmware in the RW_A &
RW_B regions with their hardcoded locations to using CBFS to find
them.  They still need to be at the hardcoded locations so that we
can set the location inside the binary, but instead of just setting
the pointer directly to them, we now search for them with cbfs.

BUG=b:154441227
TEST=Boot & verify that binaries are located in both RW-A & RW-B

Signed-off-by: Martin Roth <martin@coreboot.org>
Change-Id: I27b0593e0db7a9e6ba9b0633ac93b4d93954f002
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42831
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Eric Peers <epeers@google.com>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>