mfd: core: Add helper function to register hotplug devices

Hot-pluggable multi-function devices should always be registered with
PLATFORM_DEVID_AUTO to avoid name collisions on the platform bus. This
helper also hides the memory map and irq parameters, which aren't used
by hot-pluggable (e.g. USB-based) devices.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>