imx: add comments and remove unused struct fields

Add comment block for the imx_ddr_size function and remove the extra unused
fields from struct esd_mmdc_regs which are also not common between IMX53 and
IMX6.

Cc: Stefan Roese <sr@denx.de>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Andy Ng <andreas2025@gmail.com>
Cc: Eric Nelson <eric.nelson@boundarydevices.com>
Cc: Tapani Utriainen <tapani@technexion.com>
Cc: Tom Rini <trini@ti.com>

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Acked-by: Stefano Babic <sbabic@denx.de>
Acked-by: Nikita Kiryanov <nikita@compulab.co.il>