imx8qxp: Fix the reported CPU frequency

Currently the CPU frequency is incorrectly reported:

CPU:   NXP i.MX8QXP RevB A35 at 147228 MHz

Fix this problem by using a direct call to the SCU firmware to
retrieve the Cortex A35 CPU frequency.

With this change applied the CPU frequency is displayed correctly:

CPU:   NXP i.MX8QXP RevB A35 at 1200 MHz

Tested-by: Marcelo Macedo <marcelo.macedo@nxp.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Tested-by: Andrejs Cainikovs <andrejs.cainikovs@netmodule.com>