linux-aspeed: Move to v5.15.26 stable release

This includes security and bug fixes from the 5.15.25 through to
5.15.26 stable releases.

Notably it includes a fix for CVE-2022-0847, the "Dirty Pipe"
vulnerability.

Change-Id: I8fe3aabf8cf0bd6cf03f437a548b58f942c22b13
Signed-off-by: Joel Stanley <joel@jms.id.au>