docs: ptp.rst: add support for Renesas (IDT) ClockMatrix

Add below to “Ancillary clock features” section
  - Low Pass Filter (LPF) access from user space

Add below to list of “Supported hardware” section
  + Renesas (IDT) ClockMatrix™

Signed-off-by: Min Li <min.li.xe@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>