ARM: OMAP2+: Add structure for storing GPMC settings

The GPMC has various different configuration options such as bus-width,
synchronous or asychronous mode selection, burst mode options etc.
Currently, there is no central structure for storing all these options
when configuring the GPMC for a given device. Some of the options are
stored in the GPMC timing structure and some are directly programmed
into the GPMC configuration register. Add a new structure to store
these options and convert code to use this structure. Adding this
structure will allow us to create a common function for configuring
these options.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Tested-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>