board: stm32mp1: Fix warnings when compiling with W=1

This patch solves the following warnings:

warning: no previous prototype for 'board_quiesce_devices' [-Wmissing-prototypes]
 void board_quiesce_devices(void)
      ^~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>

board/st/stm32mp1/stm32mp1.c

@@ -4,6 +4,7 @@
  */
 #include <common.h>
 #include <adc.h>
+#include <bootm.h>
 #include <config.h>
 #include <clk.h>
 #include <dm.h>