pata: imx: add support of setting timings for PIO modes

The controller is capable to operate in up to PIO4 mode, however
before the change the driver relies on timing settings done by
a bootloader for PIO0 mode only. The change adds more flexibility
in PIO mode selection at runtime and makes the driver to work even if
bootloader does not preset ATA timings.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>