common: Drop asm/u-boot.h from common header

Move this out of the common header and include it only where needed.

There is quite a bit of clean-up that could be undertaken late. For
example, some of the function prototypes defined in the various arch's
u-boot-xxx.h header are generic and could be defined in the generic
init.h header file.

Signed-off-by: Simon Glass <sjg@chromium.org>
[trini: Only add <asm/u-boot.h> in fewer cases, PowerPC conversions]
Signed-off-by: Tom Rini <trini@konsulko.com>