dt-bindings: PCI: brcmstb: Add property "brcm,clkreq-mode"

The Broadcom STB/CM PCIe HW -- a core that is also used by RPi SOCs --
requires the driver to deliberately place the RC HW one of three CLKREQ#
modes.  The "brcm,clkreq-mode" property allows the user to override the
default setting.  If this property is omitted, the default mode shall be
"default".

Link: https://lore.kernel.org/linux-pci/20231113185607.1756-2-james.quinlan@broadcom.com
Tested-by: Cyril Brulebois <cyril@debamax.com>
Tested-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Jim Quinlan <james.quinlan@broadcom.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>