system-arm: Fix dtsi dependencies in Makefile

system-arm: Fix dtsi dependencies in Makefile

Making vexpress_gem5_vX.dtsi depend on vexpress_gem5_vX_base.dtsi
does nothing, since vexpress_gem5_vX.dtsi is never built (much in
the same way as there is no point in making a C header depend on
another).

Fix that by making all the .dts depend on both .dtsi's.

Change-Id: I9131e0b1b2e521bb09d14721dec38bf6a2d98583
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-by: Ruben Ayrapetyan <ruben.ayrapetyan@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/16143
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>