dt-bindings: pwm: renesas-tpu: Document r8a7744 support

Document r8a7744 specific compatible strings. No driver change is
needed as the fallback compatible string "renesas,tpu" activates the
right code in the driver.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Chris Paterson <Chris.Paterson2@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>