plat/palmetto: Move most code to a common file & reorder inits a bit

This moves most of the palmetto platform code to a "common" file
to share with other platforms using AST BMC.

We also initialize IPMI later.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
platform: Rename "bmc" to "astbmc"

We might have a different BMC in the future..

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
platform: Add Habanero platform

For now identical to Palmetto.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>