arm64: dts: broadcom: amend the comment about the role of BCM2712 board DTS

arm64: dts: broadcom: amend the comment about the role of BCM2712 board DTS

Current board DTS for Raspberry Pi5 states that bcm2712-rpi-5-b.dts
should not be modified and all declarations should go in the overlay
board DTS instead (bcm2712-rpi-5-b-ovl-rp1.dts).

There's a caveat though: there's currently no infrastructure to reliably
reference nodes that have not been declared yet, as is the case when
loading those nodes from a runtime overlay. For more details about
these limitations see [1] and follow-ups.

Change the comment to make it clear which DTS file will host specific
nodes, especially the RP1 related nodes which should be customized
outside the overlay DTS.

Link
[1] - https://lore.kernel.org/all/CAMEGJJ3=W8_R0xBvm8r+Q7iExZx8xPBHEWWGAT9ngpGWDSKCaQ@mail.gmail.com/

Signed-off-by: Andrea della Porta <andrea.porta@suse.com>
Link: https://lore.kernel.org/r/47f6368a77d6bd846c02942d20c07dd48e0ae7df.1754914766.git.andrea.porta@suse.com
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>