ARM: UniPhier: leave the last element of boot_device_table empty

Checking if the pointer is NULL would be easier to know the tail
of the boot_device_table[] array.
For clarification, add the /* sentinel */ comment.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/arch/arm/cpu/armv7/uniphier/ph1-pro4/boot-mode.c b/arch/arm/cpu/armv7/uniphier/ph1-pro4/boot-mode.c
index 90d4186..c31b74b 100644
--- a/arch/arm/cpu/armv7/uniphier/ph1-pro4/boot-mode.c
+++ b/arch/arm/cpu/armv7/uniphier/ph1-pro4/boot-mode.c
@@ -45,7 +45,7 @@
 	{BOOT_DEVICE_NONE, "Reserved"},
 	{BOOT_DEVICE_NONE, "Reserved"},
 	{BOOT_DEVICE_NONE, "Reserved"},
-	{BOOT_DEVICE_NONE, ""}
+	{ /* sentinel */ }
 };
 
 int get_boot_mode_sel(void)