Maintainers.txt: Add Silicon/ARM directory

Include Silicon/ARM directory under ARM section entry.

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Signed-off-by: Pranav Madhu <Pranav.Madhu@arm.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>
Reviewed-by: Thomas Abraham <thomas.abraham@arm.com>