Add description for the 7712 SoC, a Broadcom STB sibling chip of the RPi 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.
Signed-off-by: Jim Quinlan <james.quinlan@xxxxxxxxxxxx>