MIPS: kexec: Use prepare method from Generic for UHI platforms
Out-of-tree platforms may not be based on Generic as shown in customer
communication. Share the prepare method with all using UHI boot protocol,
and put into machine_kexec.c.
The benefit is that, when having kexec_args related problems, developers
will naturally look into machine_kexec.c, where "CONFIG_UHI_BOOT" will be
found, prompting them to add "select UHI_BOOT" to the platform Kconfig. It
would otherwise require a lot debugging or online searching to be aware
that the solution is in Generic code.