nds32: Remove dead reset_cpu() implementation

nds32 is one of the only architectures which still have a reset_cpu()
implementation that makes use of the `addr` parameter.  The rest of
U-Boot now ignores it and passes 0 everywhere.  It turns out that even
here, reset_cpu() is no longer referenced anywhere; reset is either not
implemented (e.g. ae3xx) or realized using a WDT (e.g. ag101).

Remove this left-over implementation in preparation for the removal of
the `addr` parameter in the entire tree.

Cc: Rick Chen <rick@andestech.com>
Signed-off-by: Harald Seiler <hws@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Rick Chen <rick@andestech.com>