Draft: arm: dts: imx93: update heartbeat gpio

A wrong GPIO was specified in U-Boot device tree for heartbeat LED for iMX93. Actualized the configuration by analogy with Linux.

But this MR does not enable a corresponding LED driver in U-Boot config, so it won't flash.

Note: both iMX8MP and iMX8MM designs are affected by this issue (on iMX8MM, this LED is LIT on power on).

Kernel MR: linux!146 (closed)