system-arm: Update gem5 timer interrupt specification

system-arm: Update gem5 timer interrupt specification

The DTB for the VExpress_GEM5_V1 was incorrectly flagging timer
interrupts as being edge triggered. Describe the interrupt as being
level triggered to match Juno and FVP.

Change-Id: I9ce4b8959e7cc28d8b208727119ff20e581311f8
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Gabor Dozsa <gabor.dozsa@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/10024
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>