trusty: Mark nxp targets as 32-bit

We rely on KERNEL_32BIT being set for Trusty targets that are 32-bit
platforms. Adds this make variable definition to the nxp targets.

Test: build.py imx7d pico7d sdb7d
Bug: None
Change-Id: Ide67ed44ef941f58c10284aa09ec4183acae7047
diff --git a/project/imx7d-inc.mk b/project/imx7d-inc.mk
index a504688..4af7572 100644
--- a/project/imx7d-inc.mk
+++ b/project/imx7d-inc.mk
@@ -19,6 +19,7 @@
 SMP_MAX_CPUS ?= 2
 SMP_CPU_CLUSTER_SHIFT ?= 1
 WITH_SMP := 1
+KERNEL_32BIT := true
 
 # select timer
 GLOBAL_DEFINES += TIMER_ARM_GENERIC_SELECTED=CNTPS