fsl-dspi: Add correct compatible string for 'spidev'

commit a62946dc2924675a12460f243dc22fefb48207d6 from
https://source.codeaurora.org/external/autobsps32/linux



In order to avoid the following warning at kernel boot:
'buggy DT: spidev listed directly in DT'

Issue: ALB-4840
Signed-off-by: Ciprian Marian Costea <ciprianmarian.costea@nxp.com>
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>