Keep boot parameters in ARM trampoline code

Currently boot parameters (r0 - r3) are forgotten in ARM trampoline code.
This patch save them at startup and restore them before jumping into kernel
_start() routine.
This is usefull when booting with Linux ABI and/or custom bootloader.

Submitted by: Grégory Soutadé <soutade@gmail.com>
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D7395