reset: add no-op stubs for optional reset control

My motivation for this patch is to make reset control handling
optional for generic drivers.

I want to add reset control to drivers/usb/host/ehci-generic.c,
but it is used by several platforms, some will implement a reset
controller driver, some will not.

Add no-op stubs in order to avoid link error for drivers that
implement reset controlling, but still it is optional.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>