[target][zynq-qemu] get the qemu target working after clock changes

The qemu target seems to emulate a 33Mhz crystal with default
pll settings.

platform/zynq/platform.c

@@ -31,8 +31,8 @@
 #include <platform/zynq.h>
 #include "platform_p.h"
 
-// target must specify this as the initial jam table to set up the soc
-extern int ps7_init(void);
+/* target can specify this as the initial jam table to set up the soc */
+__WEAK void ps7_init(void) { }
 
 void platform_init_mmu_mappings(void)
 {

target/zynq-qemu/rules.mk

@@ -7,10 +7,8 @@ GLOBAL_INCLUDES += \
 
 PLATFORM := zynq
 
-MODULES += \
-
 GLOBAL_DEFINES += \
-	TIMER_CLOCK_FREQ=100000000
+	EXTERNAL_CLOCK_FREQ=33000000
 
 #include make/module.mk