platform/x86: Move Mellanox platform hotplug driver to platform/mellanox

In preparation for making the hotplug driver build for different
architectures, move mlxcpld-hotplug.c to platform/mellanox and the
header to include/linux/platform_data as mlxreg.h to reflect the new
interface changes to come.

Replace references to CPLD with REG throughout the files, consistent
with the new name.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
[dvhart: update copyright, rewrite commit message]
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>