microblaze: turn CONFIG_OF into a select

so that we can make CONFIG_OF global and remove it from
the architecture Kconfig files later.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>