imx8qm: fix cpu frequency reporting

CPU frequency reporting failed with the following error message being
printed:

sc_pm_get_clock_rate: resource:507 clk:2: res:3
Could not read CPU frequency: -22
CPU:   NXP i.MX8QM RevB A53 at 0 MHz

Fix this by differentiating between the A35 as found on the i.MX 8QXP
and the A53 as found on the i.MX 8QM SoCs.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Max Krummenacher <max.krummenacher@toradex.com>