The compatible string "simple-bus" is well defined in ePAPR, while
I see no documentation for the "arm,amba-bus" arnywhere in ePAPR or
Documentation/devicetree/.

DT is also used by other projects than Linux kernel.  It is not a
good idea to rely on such an unofficial binding.

This commit
  - replaces "arm,amba-bus" with "simple-bus"
  - drops "arm,amba-bus" where it is used along with "simple-bus"
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Olof Johansson <olof@lixom.net>