dtc: Add data_append_integer function

This function deals with appending integers of various sizes (8, 16
32, and 64 bit currently).  It handles endianess conversions.  If the
integer will not fit in the requested number of bits of storage it
will have it's high bits ignored.

This patch also rewrites data_append_cell and data_append_addr to use
data_append_integer.

Signed-off-by: Anton Staaf <robotboy@chromium.org>
Acked-by: David Gibson <david@gibson.dropbear.id.au>