Update kernel submodule (#10411)

Update sonic-linux-kernel submodule to updated 202012 branch. This brings in the following commits....

```
e97f9fc [202012] Add upstreamed patches which backport support for registers for CPLD PNs (sonic-net#275)
58abcdc Merge pull request sonic-net#267 from Staphylo/202012-log-buf-len
3f16f4f Merge pull request sonic-net#268 from Staphylo/202012-emmc-fixes
a120ae7 Apply kernel patches to fix emmc unreliability
5f4a3f3 Increase log_buf_len to 1M for all architecture
```