docs: dt: fix documented Primecell compatible string

Only arm,primecell is documented as compatible string for Primecell
peripherals. Current code agrees with that.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Link: https://lore.kernel.org/r/9e137548c4e76e0d8deef6d49460cb37897934ca.1682333574.git.baruch@tkos.co.il


Signed-off-by: Rob Herring <robh@kernel.org>