mtd: rawnand: omap_gpmc: Enable SYS_NAND_PAGE_COUNT for OMAP_GPMC

The symbol is required for NAND support in SPL when using
OMAP_GPMC driver.

Signed-off-by: Roger Quadros <rogerq@kernel.org>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>