xen/arm: p2m: Re-implement p2m_cache_flush using p2m_get_entry

xen/arm: p2m: Re-implement p2m_cache_flush using p2m_get_entry

The function p2m_cache_flush can be re-implemented using the generic
function p2m_get_entry by iterating over the range and using the mapping
order given by the callee.

As the current implementation, no preemption is implemented, although
the comment in the current code claimed it. As the function is called by
a DOMCTL with a region of 1GB maximum, I think the preemption can be
left unimplemented for now.

Finally drop the operation CACHEFLUSH in apply_one_level as nobody is
using it anymore. Note that the function could have been dropped in one
go at the end, however I find easier to drop the operations one by one
avoiding a big deletion in the patch that convert the last operation.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Tested-by: Tamas K Lengyel <tamas@tklengyel.com>