Platform/RaspberryPi4: Correct thermal offset

The current mainline DT indicates that the thermal offset
on the rpi is 410040 rather than the 419949 being used.
This means our temp calculation is offset nearly 10C higher
when running in ACPI mode vs DT.

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>