board: imx8mp_a53: enlarge networking stack size

This patch is to fix issue #74594

The issue is caused by default stack size 1500 bytes of NET_RX_STACK_SIZE
is to small for ARM64 platform, then the stack of net_rx thread is overflow
and break the net_mgmt thread's stack, as a result net_mgmt thread is
crashed.

So this patch will enlarge all the stack size of net stack threads.

Signed-off-by: Jiafei Pan <Jiafei.Pan@nxp.com>