ARM: gemini: convert to ARMv4 multiplatform

This converts the Gemini platform to ARMv4 multiplatform, deleting
the local <mach/*> include directory, moving an idiomatic local
idling function into the .machine_init() call and getting rid of
the Makefile.boot finally.

Cc: Janos Laube <janos.dev@gmail.com>
Cc: Paulius Zaleckas <paulius.zaleckas@gmail.com>
Cc: Hans Ulli Kroll <ulli.kroll@googlemail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>