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>
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Link: https://lore.kernel.org/all/20221220102203.52398-7-rogerq@kernel.org