imx: initialize and use generic timer on i.MX 6UL/ULL

The i.MX 6UL/ULL feature a Cortex-A7 CPU which suppor the ARM
generic timer. This change makes use of the ARM generic timer in
U-Boot.

This is crucial to make the ARM generic timers usable in Linux since
timer_init() initalizes the system counter module, which is necessary
to use the generic timers CP15 registers.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>