libata: do not limit R-Car SATA driver to shmobile

The motivation for this is to allow the driver to be
used with the r8a7790 SoC.

I believe that rather than adding another SoC to the list of allowed SoCs
it is better to simply remove the dependency of the driver on shmobile all
together.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Tejun Heo <tj@kernel.org>