dtoc: Rename the phandle struct

Rather than naming the phandle struct according to the number of cells it
uses (e.g. struct phandle_2_cell) name it according to the number of
arguments it has (e.g. struct phandle_1_arg). This is a more intuitive
naming.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Kever Yang <kever.yang@rock-chips.com>