dt: remove of_alias_get_id() reference

of_alias_get_id() is broken and being reverted.  Remove the reference
to it and replace with a single incrementing id number.

There is no risk of regression here on the imx driver since the imx
change to use of_alias_get_id() is commit 22698aa2, "serial/imx: add
device tree probe support" which is new for v3.1, and it won't get
used unless CONFIG_OF is enabled and the board is booted using a
device tree.  A single incrementing integer is sufficient for now.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Shawn Guo <shawn.guo@linaro.org>