vc/google/chromeos: Deprecate support for SAR tables in VPD

vc/google/chromeos: Deprecate support for SAR tables in VPD

SAR table in VPD has been deprecated for Chrome OS platforms for > 1
year now. All new Chrome OS platforms have switched to using SAR
tables from CBFS.

This change drops the support for SAR table in VPD from coreboot to
align with the factory changes. `get_wifi_sar_limits()` is thus
updated to look for SAR file in CBFS only.

Anyone building ToT coreboot for an already released Chrome OS
platform with SAR table in VPD will have to extract the "wifi_sar" key
from VPD and add it as a file to CBFS using following steps:

 - On DUT, read SAR value using `vpd -i RO_VPD -g wifi_sar`
 - In coreboot repo, generate CBFS SAR file using:
   `echo ${SAR_STRING} > site-local/${BOARD}-sar.hex`
 - Add to site-local/Kconfig:
   ```
   config WIFI_SAR_CBFS_FILEPATH
     string
     default "site-local/${BOARD}-sar.hex"
   ```

BUG=b:173465272

Change-Id: I21d190dcc9f3554fab6e21b4498e7588a32bb1f0
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51483
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>