dt-bindings: usb: max3421: Interrupt-parent is optional

Documentation modification, now interrupt-parent is an optional
property. Also fix few typos.

Signed-off-by: Jules Maselbas <jules.maselbas@grenoble-inp.org>
Reported-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>