xen/arm: Allow Xen to run on multiple platform without recompilation

xen/arm: Allow Xen to run on multiple platform without recompilation

Xen can include various platform support (ie: exynos5, versatile express...)
and choose during boot time a set of callbacks for the current board.
These callbacks will be called in places where each board can have specific
code. For the moment the callbacks are:
    - platform_init: additional initialization for the platform
    - platform_init_time: some platform (ie: Exynos 5) needs to initialize
    the timer with an uncommon way
    - platform_specific_mapping: add mapping to dom0 which are not specified
    in the device tree
    - platform_reset: reset the platform
    - platform_poweroff: poweroff the platform
    - platform_quirks: list of quirks for a specific board.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>