ARM: OMAP2+: gpmc: Introduce gpmc_set_legacy()

Introduce a helper function to complete the setting of some GPMC
parameters, only used when the gpmc is probed from a board file.
As such, it will go away once the DT conversion is completed.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>