ARM: OMAP2+: nand: reorganize gpmc timing values

This patch removes omap2_nand_gpmc_retime() which was used to quirk
some timing values before gpmc_cs_set_timings(). Due to recent changes,
gpmc_cs_set_timings() has evolved so that there is no more need for a
retime function.

To keep the gpmc configuration consistent for legacy board files, this
patch also adds oe_on and we_on to nand_default_timings[] as they
would be by the retime function.

Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>