serial: sh-sci: use of_property_read_bool()

Use more compact of_property_read_bool() call for a boolean property
instead  of of_find_property() call in sci_parse_dt().

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>