rpi: docs: Update maintainers file to new RPi directory scheme

With the addition of the Raspberry Pi 4 port the directory structure
changed a bit, also the new port didn't have a separate entry.

Add a new entry for the RPi4 port and adjust the path names.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: I04b60e729a19bb0cc3dd6ce6899ec6480356b1f1
1 file changed