arm: spear: Move to common SPL infrastructure

arm: spear: Move to common SPL infrastructure

The SPL implementation for SPEAr600 is older than the common SPL
infrastructure. This patch now moves the SPEAr600 SPL over to the
common SPL code.

Tested on the only SPEAr board that currently uses SPL in mainline
U-Boot, the x600.

Signed-off-by: Stefan Roese <sr@denx.de>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Vipin Kumar <vk.vipin@gmail.com>