ARM: imx: fix imx6sll-only build

ARM: imx: fix imx6sll-only build

When selecting SOC_IMX6SLL but not SOC_IMX6SL, we get a link error:

arch/arm/mach-imx/mach-imx6sl.o: In function `imx6sl_init_late':
mach-imx6sl.c:(.init.text+0x14): undefined reference to `imx6sl_cpuidle_init'

This adds the missing line to the Makefile to also build the cpuidle
support that we need here.

Fixes: 1b012d5413db ("ARM: imx: Add basic msl support for imx6sll")
Acked-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>