dev-arm: Add new VExpress_GEM5_V1_Base Platform

dev-arm: Add new VExpress_GEM5_V1_Base Platform

Add a new VExpress_GEM5_V1_Base Platform which configures basic on
chip devices. The original VExpress_GEM5_V1 will inherit the Base and
add more on chip devices (currently only the HDLCD). This change will
make it possible to create variations of the base platform with
different devices.

Change-Id: I21f9bf4f6217d87e811ff777f630122593eef013
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/10807
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>