ACPI: EC: Add ec_get_handle()

toshiba_acpi needs to execute an AML method within the EC namespace
to make hotkeys work on some platforms. Provide an interface to
allow it to easily get a handle to the EC namespace for this purpose.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>