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
Acked-by: Andy Shevchenko
[dvhart: update copyright, rewrite commit message]
Signed-off-by: Darren Hart (VMware)