MAINTAINERS: Update EEH details and maintainership

Enhanced Error Handling could mean anything in the context of the entire
kernel, so change the name to reference that it is both for PCI and
powerpc.

EEH covers a bit more than the previously listed files, so add the headers
and platform-specific code to the EEH maintained section.

In addition, I am taking over the maintainership.

Signed-off-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>