ARM: dts: qcom: apq8064-ifc6410 Use hardware flow control for GSBI6

GSBI6 UART module is connected to BT chip, which uses
hardware flow control lines. Enable them on SoC side.

Signed-off-by: Ivan T. Ivanov <ivan.ivanov@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>