Introduce fdtw_read_array() helper

fdtw_read_cells() can only read one or two cells, sometimes it may be
needed to read more cells from one property.

Change-Id: Ie70dc76d1540cd6a04787cde7cccb4d1bafc7282
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>