Kexec: Add ram_console support.

- Add reserve function to machine_desc to reserve ram_console memory.
- Place in the last 1 MB of RAM, in high memory.
- Exclude the final 4 kB for the kexec hardboot page.

credits to Mike Kasick as he added this support for Exynos devices.
Kexec: Enable and fix kexec syscall support.

Avoid second flush_cache_all() call in machine_kexec() as it causes a CPU
hang on Exynos 4.

The suggestion to call cpu_reset by its physical address comes from an
earlier kexec support patch by Will Deacon [1].

[1] W. Deacon. [4/6] ARM: reset: add reset functionality for jumping to a
    physical address, Jun. 2011. https://patchwork.kernel.org/patch/852802/

Credits to Mike Kasick.