ARM: dts: bcm2711-rpi-4-b: Add gpio offsets to line name array

this helps human readers considerably to determine the line name for a
given offset or vice versa.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
[ nsaenz: corrected patch title ]
Signed-off-by: Nicolas Saenz Julienne <nsaenz@kernel.org>
Link: https://lore.kernel.org/r/20211130161147.317653-1-u.kleine-koenig@pengutronix.de