pico-imx6ul: Convert to SPL

pico-imx6ul: Convert to SPL

There are two versions of imx6ul pico SOMs: one with 256MB and another
one with 512MB of RAM.

Convert to SPL so that both versions can be supported. This patch
doesn't rework the clock initialization to avoid changing the behavior
in this same patch, so it will be cleaned up in future.

Currently only the 256MB is tested/supported.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Fabio Berton <fabio.berton@ossystems.com.br>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>