ARM: imx: move revision definitions and declarations into a header

The revision definitions and declarations are widely used by clock
drivers.  As a step of moving clock drivers out of arch/arm/mach-imx,
let's create header include/soc/imx/revision.h to accommodate them.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>

arch/arm/mach-imx/mx3x.h

@@ -185,11 +185,4 @@
 
 #define MX3x_PROD_SIGNATURE		0x1	/* For MX31 */
 
-/* Mandatory defines used globally */
-
-#if !defined(__ASSEMBLY__) && !defined(__MXC_BOOT_UNCOMPRESS)
-extern int mx35_revision(void);
-extern int mx31_revision(void);
-#endif
-
 #endif /* ifndef __MACH_MX3x_H__ */