meson: add svga gallium driver to 'auto' group on arm/aarch64

svga gallium driver is a good citizen on arm/aarch64 linux targets. Enable that driver for -Dgallium-drivers=auto builds.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: Eric Engestrom <eric@igalia.com>
(cherry picked from commit 6e311dd7)

\cc @dbaker