MIPS: ath79: Remove some unused code from setup.c

Remove the unused defines for the reference clocks rate
and the useless machine init function.

Signed-off-by: Alban Bedel <albeu@free.fr>
Cc: Felix Fietkau <nbd@openwrt.org>
Cc: Qais Yousef <qais.yousef@imgtec.com>
Cc: Andrew Bresticker <abrestic@chromium.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/11505/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>