snapdragon: added msm_board_serial() func

This commit adds a function to get the board
serial number.
In snapdragon it's actually the eMMC serial number.

Function added in a new file misc.c that will
include further snapdragon miscellaneous functions.

Signed-off-by: Ramon Fried <ramon.fried@gmail.com>
diff --git a/arch/arm/mach-snapdragon/Makefile b/arch/arm/mach-snapdragon/Makefile
index f375d07..2d94083 100644
--- a/arch/arm/mach-snapdragon/Makefile
+++ b/arch/arm/mach-snapdragon/Makefile
@@ -8,5 +8,6 @@
 obj-$(CONFIG_TARGET_DRAGONBOARD410C) += sysmap-apq8016.o
 obj-$(CONFIG_TARGET_DRAGONBOARD410C) += pinctrl-apq8016.o
 obj-$(CONFIG_TARGET_DRAGONBOARD410C) += pinctrl-snapdragon.o
+obj-y += misc.o
 obj-y += clock-snapdragon.o
 obj-y += dram.o