ARM: Fix build if CLKDEV_LOOKUP is not configured

mips:allmodconfig fails to build with

drivers/clocksource/timer-sp804.c: In function '__sp804_clocksource_and_sched_clock_init':
drivers/clocksource/timer-sp804.c:88:3: error: implicit declaration of function 'clk_get_sys'

because CLKDEV_LOOKUP is not configured and the driver depends on it.

Fixes: 0b7402dc

 ("ARM: 8366/1: move Dual-Timer SP804 driver to drivers/clocksource")
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>