There are cases where we have multiple device instances
connected to a single GPMC chip select. For example, there
are four UARTs on the Zoom debug boards that all share a
single chip select and a GPIO interrupt.

We do have support for this already in theory, but it's broken
because we're bailing out if the chip select is already taken.

To be able to provide checks on the chip select usage, let's
add new struct gpmc_cs_data so we can start using already
registered device names for checks.

Later on we probably want to start using struct gpmc_cs_data
as a wrapper for all the GPMC chipselect related data.
Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>