fdt_support: add 'const' qualifier for unchanged argument

In the next commit, I will add a new function, fdt_pack_reg()
which uses get_cells_len().

Beforehand, this commit adds 'const' qualifier to get_cells_len().
Otherwise, a warning message will appear:
 warning: passing argument 1 of 'get_cells_len' discards 'const'
 qualifier from pointer target type [enabled by default]

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>