ARM:AM33XX: Add SPL support for AM335X EVM

This patch is added to support SPL feature on AM335X
platform. In this patch, MMC1 is configured as boot
device for SPL and support for other devices will be
added in the next patch series.

Signed-off-by: Chandan Nath <chandan.nath@ti.com>
Signed-off-by: Tom Rini <trini@ti.com>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>