drm/bridge: tc358768: Clean up clock period code

The driver defines TC358768_PRECISION as 1000, and uses "nsk" to refer
to clock periods. The original author does not remember where all this
came from. Effectively the driver is using picoseconds as the unit for
clock periods, yet referring to them by "nsk".

Clean this up by just saying the periods are in picoseconds.

Reviewed-by: Peter Ujfalusi <peter.ujfalusi@gmail.com>
Tested-by: Maxim Schwalm <maxim.schwalm@gmail.com> # Asus TF700T
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230906-tc358768-v4-10-31725f008a50@ideasonboard.com