event: Convert arch_cpu_init_dm() to use events

Instead of a special function, send an event after driver model is inited
and adjust the boards which use this function.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 28234aa..06cae68 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -130,6 +130,7 @@
 config TARGET_BOSTON
 	bool "Support Boston"
 	select DM
+	imply DM_EVENT
 	select DM_SERIAL
 	select MIPS_CM
 	select SYS_CACHE_SHIFT_6