OvmfPkg/PlatformInitLib: Add PlatformAddHobCB

Add PlatformAddHobCB() callback function for use with
PlatformScanE820().  It adds HOBs for high memory and reservations (low
memory is handled elsewhere because there are some special cases to
consider).  This replaces calls to PlatformScanOrAdd64BitE820Ram() with
AddHighHobs = TRUE.

Write any actions done (adding HOBs, skip unknown types) to the firmware
log with INFO loglevel.

Also remove PlatformScanOrAdd64BitE820Ram() which is not used any more.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>