mtd: rawnand: davinci: Reduce polling interval in NAND_OP_WAITRDY_INSTR

For each NAND_OP_WAITRDY_INSTR operation, the NANDFSR register is
polled only once every 100 us to check for the EMA_WAIT pin. This
isn't frequent enough and causes delays in NAND accesses.

Set the polling interval to 5 us. It increases the page read speed
reported by flash_speed by ~30% (~10% on page writes).

Signed-off-by: Bastien Curutchet <bastien.curutchet@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>