ARM: dts: bcm2838-rpi-4-b: Backport BT part from upstream

The CYW43455 on the Raspberry Pi 4 doesn't use an external pin as lower
power clock anymore. So drop the GPIO clock from pinctrl. While we are at
this add the missing declaration of hardware flow control.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>