board/qemu/arm-vexpress-tz: use enable=on

Since Qemu 6.0.0 [1], a warning appear in the log if a short-form
boolean option is used.

[1] https://git.qemu.org/?p=qemu.git;a=commitdiff;h=ccd3b3b8112b670fdccf8a392b8419b173ffccb4

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Etienne Carriere <etienne.carriere@linaro.org>
Cc: Dick Olsson <hi@senzilla.io>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

board/qemu/arm-vexpress-tz/readme.txt

@@ -16,7 +16,7 @@ from the image directory:
 	-smp 1 -s -m 1024 -d unimp \
 	-serial stdio \
 	-netdev user,id=vmnic -device virtio-net-device,netdev=vmnic \
-	-semihosting-config enable,target=native \
+	-semihosting-config enable=on,target=native \
 	-bios flash.bin # qemu_arm_vexpress_tz_defconfig
 
 The boot stage traces (if any) followed by the login prompt will appear