ARM: imx: call imx6sx_cpuidle_init() conditionally for 6sll

The imx6sl platform has two different cpuidle implementations,
and fails to link if we only want one of the two:

arch/arm/mach-imx/mach-imx6sl.o: In function `imx6sl_init_late':
mach-imx6sl.c:(.init.text+0x12): undefined reference to `imx6sx_cpuidle_init'

This makes the call into reference conditional on the configuration.

Fixes: e7fa1fb3 ("ARM: imx: add cpu idle support for i.MX6SLL")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>