imx: Align the imximage header and payload to multiples of 4k

The MX53 ROM loads the data from NAND in multiples of pages and
supports maximum page size of 4k. Thus, align the image and header
to 4k to be safe from ROM bugs.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Cc: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tom Rini <trini@ti.com>