pwm: jz4740: Remove unused devicetree compatible strings

Right now none of the Ingenic-based boards probe this driver from
devicetree. This driver defined three compatible strings for the exact
same behaviour. Before these strings are used, we can remove two of
them.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>