platform/x86: mlx-platform: make a couple of structures static

The structures mlxplat_dev and mlxplat_hotplug are local to the source
and do not need to be in global scope, so make them static.

Cleans up sparse warnings:
symbol 'mlxplat_dev' was not declared. Should it be static?
symbol 'mlxplat_hotplug' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>