tty/bcm63xx_uart: allow naming clock in device tree

tty/bcm63xx_uart: allow naming clock in device tree

Codify using a named clock for the refclk of the uart. This makes it
easier if we might need to add a gating clock (like present on the
BCM6345).

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Kevin Cernekee <cernekee@gmail.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: linux-mips@linux-mips.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-serial@vger.kernel.org
Cc: devicetree@vger.kernel.org
Cc: bcm-kernel-feedback-list@broadcom.com
Patchwork: https://patchwork.linux-mips.org/patch/17328/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: James Hogan <jhogan@kernel.org>