MAINTAINERS: Remove powerpc's "opal" pattern match

The block guys have merged a new driver called "sed-opal", which is
getting caught by our pattern match on "opal".

So drop our "opal" pattern and replace it with matches against the
specific paths for our drivers. We should try and remember to add new
drivers as they're added in future.

Suggested-by: Jon Derrick <jonathan.derrick@intel.com>
Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
[mpe: Fix ic2/i2c typo, write change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>