dev-arm: Rename GIC maintenance interrupt from ppint to maint_int

dev-arm: Rename GIC maintenance interrupt from ppint to maint_int

ppint is a generic name which only reflects Arm recommendation of
assigning the maintanance interrupt to a PPI (numbered 25)

Change-Id: Ic5abb6ed50817ad2d165b5df46dd989eb195a9db
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Anouk Van Laer <anouk.vanlaer@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17628
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>