fastmodel: Fix compilation errors

fastmodel: Fix compilation errors

This changes fixes two compilation errors when compiling with
FastModels. One is that CurrentMsn should be Iris::CurrentMsn and the
other is that currEL() function needs arch/arm/utility.hh header file.

Test by compiling GEM5 with FastModels:
scons -j64 build/ARM/gem5.opt \
  USE_ARM_FASTMODEL=1     \
  PVLIB_HOME=...          \
  MAXCORE_HOME=...        \
  ARMLMD_LICENSE_FILE=... \

Change-Id: Iabe0a5f25246591f99b57219428b8f87ecd3363c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23924
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>