drivers: net: Move Ethernet device definition for SLIP to ethernet

Currently, if CONFIG_SLIP_TAP is enabled (default for QEMU), and no
other Ethernet driver is enabled, the following warning is printed by
CMake:
    No SOURCES given to Zephyr library: drivers__ethernet

This happens because SLIP_TAP enabled Ethernet L2, but has no actual
sources in drivers/ethernet. This commit fixes this, by moving the
actual definition of the SLIP TAP Ethernet interface into a separate
file, within drivers/ethernet. Technically, in that configuration SLIP
defines a Ethernet device, implementing Ethernet API, so such a change
is justifiable, and prevents unwanted warning from being generated.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>