[MIPS] Change get_cycles to always return 0.

This avoids us executing an mfc0 c0_count instruction on processors which
don't have but also on certain R4000 and R4400 versions where reading from
the count register just in the very moment when its value equals
c0_compare will result in the timer interrupt getting lost.

There is still a number of users of get_cycles remaining outside the
arch code:

crypto/tcrypt.c:                start = get_cycles();
crypto/tcrypt.c:                end = get_cycles();
crypto/tcrypt.c:                start = get_cycles();
crypto/tcrypt.c:                end = get_cycles();
crypto/tcrypt.c:                start = get_cycles();
crypto/tcrypt.c:                end = get_cycles();
drivers/char/hangcheck-timer.c: return get_cycles();
drivers/char/hangcheck-timer.c: printk("Hangcheck: Using get_cycles().\n");
drivers/char/random.c:  sample.cycles = get_cycles();
drivers/input/joystick/analog.c:#define GET_TIME(x)     do { x = get_cycles(); }
include/linux/arcdevice.h:          _x = get_cycles(); \
include/linux/arcdevice.h:          _y = get_cycles(); \
mm/slub.c:      if (!s->defrag_ratio || get_cycles() % 1024 > s->defrag_ratio)
mm/slub.c:      p += 64 + (get_cycles() & 0xff) * sizeof(void *);

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