imx93-u-boot: Describe the CPU clocks in the devicetree

Currently, there is an error when the i.MX93 CPU frequency is
read:

Could not read CPU frequency: -2
CPU:   NXP i.MX93(52) Rev1.1 A55 at 0 MHz

Fix it by describing the A55 clock nodes in the devicetree, like done
on other i.MX SoCs.

With this change, the CPU frequency error is gone and it can be correctly
retrieved:

CPU:   NXP i.MX93(52) Rev1.1 A55 at 1700 MHz
CPU:   Industrial temperature grade  (-40C to 105C) at 35C

As the upstream imx93.dtsi does not describe the CPU clocks,
keep the clock node in imx93-u-boot.dtsi for now.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>