[ARM] remove useless config option GENERIC_BUST_SPINLOCK

Remove the apparently useless config option GENERIC_BUST_SPINLOCK,
since nothing in the source tree refers to it.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>