MAINTAINERS: fix bad sdei paths

The SDEI header files had an 'arm_' namespace added, but the patterns
in the MAINTAINERS files were missed. Oops.

Reported-by: Joe Perches <joe@perches.com>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>