of/flattree: eliminate cell_t typedef

A cell is firmly established as a u32.  No need to do an ugly typedef
to redefine it to cell_t.  Eliminate the unnecessary typedef so that
it doesn't have to be added to the of_fdt header file

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Tested-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>