arm64: configs: enable FlexTimer alarm timer

This driver is used on Layerscape SoCs to wake up the system from
standby. It works in conjunction with the RCPM driver. The latter is
only available as a builtin.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>