uboot-imx

Add architecture-specific global data

We plan to move architecture-specific data into a separate structure so
that we can make the rest of it common.

As a first step, create struct arch_global_data to hold these fields.
Initially it is empty.

This patch applies to all archs at once. I can split it if this is really
a pain.

Signed-off-by: Simon Glass <sjg@chromium.org>