mx6: soc: Include <asm/bootm.h> header file

When compiling with W=1 the following warning is observed:

arch/arm/mach-imx/mx6/soc.c:213:12: warning: no previous prototype for ‘get_board_rev’ [-Wmissing-prototypes] u32 __weak get_board_rev(void)

Fix the build warning by including <asm/bootm.h>.

Signed-off-by: Diego Dorta <diego.dorta@nxp.com>