ARM: rpi: support model A+

Add a board rev entry for the new model A+, and augment the board
rev error handling code to be a bit more verbose.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
diff --git a/arch/arm/include/asm/arch-bcm2835/mbox.h b/arch/arm/include/asm/arch-bcm2835/mbox.h
index 0289ba6..88d2ec1 100644
--- a/arch/arm/include/asm/arch-bcm2835/mbox.h
+++ b/arch/arm/include/asm/arch-bcm2835/mbox.h
@@ -140,6 +140,7 @@
 #define BCM2835_BOARD_REV_B_REV2_f	0xf
 #define BCM2835_BOARD_REV_B_PLUS	0x10
 #define BCM2835_BOARD_REV_CM		0x11
+#define BCM2835_BOARD_REV_A_PLUS	0x12
 
 struct bcm2835_mbox_tag_get_board_rev {
 	struct bcm2835_mbox_tag_hdr tag_hdr;