arm: maxbcm: Enable SPL to include DDR training code into U-Boot

This patch adds SPL support to the maxbcm MV78460 based board. Including
the fixed DDR configuratrion needed for the DDR training code. And the
the serdes PHY init code.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Luka Perkov <luka.perkov@sartura.hr>