ARM: sa11x0: assabet/neponest: create neponset device in assabet.c

The neponset board is a daughter board for the Assabet.  Create the
neponset platform device in assabet.c, where we don't have to wrap
it with machine_is_assabet() stuff.  We also create this device
dynamically rather than keeping it as a static device.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>