arm64: configs: Add virtconfig

arm64: configs: Add virtconfig

Provide a slimline configuration intended to be booted on virtual
machines, with the goal of providing a light configuration which will
boot on and enable features available in mach-virt.  This is defined in
terms of the standard defconfig, with an additional virt.config fragment
which disables options unneeded in a virtual configuration.

As a first step we just disable all the ARCH_ configuration options,
disabling the build of all the SoC specific drivers.  This results in a
kernel that builds about 25% faster in my testing, if this approach
works for people we can add further options.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230203-arm64-defconfigs-v1-3-cd0694a05f13@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>