mtd: gpmi: do not get the clock frequency in gpmi_begin()

The current code will gets the clock frequency which is used by
gpmi_nfc_compute_hardware_timing(). It makes the code a little mess.

So move the `get clock frequency` code to the
gpmi_nfc_compute_hardware_timing() itself. This makes the code tidy
and clean.

This patch also uses the macro NSEC_PER_SEC to replace the `1000000000`.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>