Platform: Implement UART uninitialize function

This patch introduces an uninitialize function for the UART
peripheral to restore it's default state before passing
execution to runtime image from MCUBoot.

Change-Id: Iaf239b575be82402b9c142e990fb0ea30ffe9a88
Signed-off-by: David Vincze <david.vincze@arm.com>
diff --git a/bl2/ext/mcuboot/bl2_main.c b/bl2/ext/mcuboot/bl2_main.c
index 52dece2..6b8c090 100644
--- a/bl2/ext/mcuboot/bl2_main.c
+++ b/bl2/ext/mcuboot/bl2_main.c
@@ -77,6 +77,8 @@
                                          rsp->br_hdr->ih_hdr_size);
     }
 
+    stdio_uninit();
+
     __disable_irq();
     __set_MSP(vt->msp);
     __DSB();