ARM: dts: bcm2711: Add reserved memory template to hold firmware configuration

RPi4's co-processor will copy the board's bootloader[1] configuration
into memory for the OS to consume. Specifically, for the bootloader
configuration and upgrade user-space routines to query it through
nvmem's sysfs interface.

Introduce a reserved-memory area template for the co-processor to edit
before booting the system so as for Linux not to overwrite that memory
and to expose it as an nvmem device.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Tested-by: Tim Gover <tim.gover@raspberrypi.com>
Link: https://lore.kernel.org/r/e8ca9365-a1f2-1f9d-377c-13bf97883cce@linaro.org

[1] https://www.raspberrypi.org/documentation/hardware/raspberrypi/bcm2711_bootloader_config.md