ARM: dts: Add nvmem node for BCM2711 bootloader public key

Make a copy of the bootloader secure-boot public key available to the OS
via an nvmem node. The placement information is populated by the
Raspberry Pi firmware if a public key is present in the BCM2711
bootloader EEPROM.

Signed-off-by: Tim Gover <tim.gover@raspberrypi.com>