nhk8815: remove platform.S, which was unused at link time

This source file, which I got by the vendor in their own port,
was not actually executing because lib-based compilation
didn't call lowlevel_init (we have CONFIG_SKIP_LOWLEVEL_INIT).
With the change to object-based linking, an undefined symbol in
this file started hitting in the final link.

Signed-off-by: Alessandro Rubini <rubini@unipv.it>
Acked-by: Andrea Gallo <andrea.gallo@stericsson.com>