MIPS: Introduce support for Platform definitions

Move platform specific definitions to the platfrom directories.

Each platform shall do the following:
1) include an entry in arch/mips/Kbuild.platforms
2) add relevant definitions to arch/mips/<platform>/Platform

This commits changes ar7 to the new scheme as an example.

Introducing a platform speecific Platfrom file has following advantages:
1) decentralization of platfrom definitions
2) simplification af arch/mips/Makefile
3) force all platfrom to build with -Werror (done in arch/mips/Kbuild)

[Ralf: Remove forgotten -Werror from AR7 Makefile]

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
To: linux-mips <linux-mips@linux-mips.org>
To: Wu Zhangjin <wuzhangjin@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/1302/
Patchwork: http://patchwork.linux-mips.org/patch/1308/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>