ARM: keystone: drop useless HAVE_SCHED_CLOCK

The Kconfig symbol HAVE_SCHED_CLOCK got removed in v3.4,
with commit 6905a65879b5 ("ARM: Make the sched_clock
framework mandatory"). But a select statement for it popped up again
through commit 828989ad87af ("ARM: keystone: Add minimal TI Keystone
platform support"). Drop that statement, as it is useless.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
[santosh.shilimkar@ti.com: Minor edit in the subject]
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>