MIPS: AR7: remove orphan declarations from arch/mips/include/asm/mach-ar7/ar7.h

All uses of ar7_cpu_clock, ar7_bus_clock and ar7_dsp_clock were
removed by commit 780019dd ("MIPS: AR7: Implement clock API"),
so remove the orphan declarations, too.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>