MAINTAINERS: Add Mahesh J Salgaonkar as EEH maintainer

MAINTAINERS: Add Mahesh J Salgaonkar as EEH maintainer

Update EEH entry:

  - Russell: lacks time to maintain EEH.

  - Oliver: lacks time & hardware to do actual maintenance, but happy to
    field questions and review things.

  - Mahesh: glad to take over EEH maintenance.

[bhelgaas: commit log, add Mahesh, make Oliver reviewer]
Link: https://lore.kernel.org/r/20220806085301.25142-1-ruscur@russell.cc
Signed-off-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>