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 <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Reviewed-by: Gabor Dozsa <4c5a2daf99dab97d3a4a458d9ee9896239b35fd4@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/10024
Reviewed-by: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>

TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu