gw_ventana: Use the generic spl_sd.cfg

gw_ventana can boot from SPI or NAND and both of these interfaces boot from
the same 0x400 offset.

This means that we could simplify the code and replace the custom gw_ventana.cfg
with the generic spl_sd.cfg, as it provides the same boot offset of 0x400.

Cc: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>