Hi Steve,
Can you check that the master oscillator (OSC0) goes off and DDR CKE signal goes low on suspend ?
Do you have a JTAG debugger to connect to M3 for debugging ? The PRCM register space is on L4_WKUP peripheral memory map and can be accessible from M3 when it's active. How did you check M3 received an interrupt by a wakeup event.
Suspend/Resume works with the SDK on the GP EVM. Do you have a TI GP EVM you could use it as reference ?