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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/usage-model.rst

···
 because it must decide whether to register each node as either a
 platform_device or an amba_device.  This unfortunately complicates the
 device creation model a little bit, but the solution turns out not to
-be too invasive.  If a node is compatible with "arm,amba-primecell", then
+be too invasive.  If a node is compatible with "arm,primecell", then
 of_platform_populate() will register it as an amba_device instead of a
 platform_device.