dev-arm: Correct cast of template parameter

dev-arm: Correct cast of template parameter

Clang with -Wconstant-conversion is _very_ restrictive on casting.
The shift operator results in an incorrect promotion.

This patch add a compile-time static cast that remove the error
when clang is used.

Change-Id: I3aa1e77da2565799feadc32317d5faa111b2de86
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17308
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>