mach-snapdragon: Update fdtfile logic to work for RB1 and RB2

mach-snapdragon: Update fdtfile logic to work for RB1 and RB2

RB1 and RB2 have three root compatibles where the last one can't be used
to decode fdtfile name (qcm* vs qrb*). So rather just rely on the first
compatible to retrieve the SoC name.

Signed-off-by: Sumit Garg <sumit.garg@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250505124333.12344-1-sumit.garg@kernel.org
Signed-off-by: Casey Connolly <casey.connolly@linaro.org>