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>

[1] https://www.raspberrypi.org/documentation/hardware/raspberrypi/bcm2711_bootloader_config.md
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Changes since v1:
 - Introduce compatible string
 - Change alias name to something more explicit