dt-bindings: PCI: brcm,stb-pcie: Add 7712 SoC description

dt-bindings: PCI: brcm,stb-pcie: Add 7712 SoC description

Add description for the 7712 SoC, a Broadcom STB sibling chip
of the Raspberry Pi 5.

The 7712 uses three reset controllers: rescal, for PHY reset
calibration; bridge, for the bridge between the PCIe bus and
the memory bus; and swinit, which is a "soft" initialization
of the PCIe HW.

Link: https://lore.kernel.org/linux-pci/20240815225731.40276-4-james.quinlan@broadcom.com
Signed-off-by: Jim Quinlan <james.quinlan@broadcom.com>
[kwilczynski: commit log]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>