ARM: OMAP2/3: Add generic smc91x support when connected to GPMC

Convert the board-rx51 smc91x code to be generic and make
the boards to use it. This allows future recalculation of the
timings when the source clock gets scaled.

Also correct the rx51 interrupt to be IORESOURCE_IRQ_HIGHLEVEL.

Thanks to Paul Walmsley <paul@pwsan.com> for better GPMC timing
calculations.

Signed-off-by: Tony Lindgren <tony@atomide.com>