dt-bindings: timer: ostm: Add R7S9210 support

The R7S9210 belongs to the RZ/A2 SoC series

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Rob Herring <robh@kernel.org>