shields: Add arduino uno click shield

The shield converts Arduino UNO R3 headers to two mikroBUS
sockets.
Two mickoBUS sockets are exposed via the overlay labelled
mikrobus_header_1 and mikrobus_header_2. mikrobus_header_1
is used as the default and assigned the node label mikrobus_header.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>