gdbsx: prefer privcmd character device

gdbsx: prefer privcmd character device

Prefer using the character device over the proc file if the character
device exists.

CC: Elena Ufimtseva <elena.ufimtseva@oracle.com>
CC: Ian Jackson <ian.jackson@eu.citrix.com>
CC: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Doug Goldstein <cardoe@cardoe.com>
Reviewed-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
Release-acked-by: Julien Grall <julien.grall@linaro.org>