mtd: nand: davinci_nand: update write_page function for keystone RBL

After mtd was synced with Linux 3.14
(ff94bc40)
the number of parameters for write_page function of nand_chip was
changed. The additional two var were needed for subpage write.
As keystone has no supbage write they are not needed. So correct
only function definition by upgrading it's parameter list.
That helps to get ritd of compilation warning.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>