dm: core: Introduce dev_read_alias_highest_id()

It is wrapper for calling of_alias_get_highest_id() when live tree is
enabled and fdtdec_get_alias_highest_id() if not.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>