imx: mx7: Fix CONFIG_SERIAL_TAG compilation

Currently when we define CONFIG_SERIAL_TAG we will barf with a failure to
define "struct tag_serialnr".

This structure is defined in <asm/setup.h>, this patch includes
<asm/setup.h> to fix.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>