ARM: dts: atlas7: Use generic uart-has-rtscts DT property

As of commit 7f60830a ("serial: sirf: Use generic
uart-has-rtscts DT property property"), the SiRF UART driver recognizes
the generic "uart-has-rtscts" DT property, deprecating the
vendor-specific "sirf,uart-has-rtscts" DT property. Hence replace the
latter by the former in all DTS files.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>