arm64: Enable TIMER_IMX_SYS_CTR for ARCH_MXC platforms

ARCH_MXC platforms needs system counter as broadcast timer
to support cpuidle, enable it by default.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>