MIPS: ath79: Remove unused include <asm/mips_machine.h>

Since commit 3a77e0d7 ("MIPS: ath79: drop machfiles"), this header
is not used anymore.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>