ARM: mvebu: armada-xp: Fix 74hc595 count for Lenovo ix4-300d

There is only one 74hc595 connected to GPIO but two were given
in the registers-number property. Fix it up.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Tested-By: Benoit Masson <yahoo@perenite.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>