Documentation: ABI: add sysfs interface for ZynqMP CSU registers

Document the new sysfs interface that exposes Configuration Security
Unit (CSU) registers through the zynqmp-firmware driver.

The interface is available under:

  /sys/devices/platform/firmware:zynqmp-firmware/csu_registers/

The CSU registers are discovered at boot time using the PM_QUERY_DATA
firmware API. The following registers are currently supported:

  - multiboot     (CSU_MULTI_BOOT)
  - idcode        (CSU_IDCODE, read-only)
  - pcap-status   (CSU_PCAP_STATUS, read-only)

Read operations use the existing IOCTL_READ_REG firmware interface,
while write operations use IOCTL_MASK_WRITE_REG.

Access control is enforced by the firmware. Write attempts to
read-only registers are rejected by firmware even though the sysfs file
permissions allow writes.

Document the ABI entry accordingly.

Signed-off-by: Ronak Jain <ronak.jain@amd.com>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/20260520093654.3303917-2-ronak.jain@amd.com

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