libefi: add efi_devpath_next_instance()

UEFI device path may be path to one device, or concatenated list of instances
to different devices (textually represented as comma separated list).

Provide generic function to get next instance from device path.
Returns next instance or end node.

The use case is like:

EFI_DEVICE_PATH *node = (EFI_DEVICE_PATH *)buf;
while (!IsDevicePathEnd(node)) {
process(node);
node = efi_devpath_next_instance(node);
}

Where buf is pointing to either single device path or
concatenated list of device paths (such as from ConIn or ConOut).

Reviewers: imp
Differential Revision: https://reviews.freebsd.org/D40081