imx8m: add NUM_PPI

This allows SMP support for the imx8m.

src/plat/imx8m-evk/config.cmake

@@ -41,6 +41,7 @@ if(KernelPlatformImx8mq-evk OR KernelPlatformImx8mm-evk)
         MAX_IRQ 160
         TIMER drivers/timer/arm_generic.h
         INTERRUPT_CONTROLLER arch/machine/gic_v3.h
+        NUM_PPI 32
     )
 endif()