xen: arm: clarify cacheability requirements of hypercall arguments.

xen: arm: clarify cacheability requirements of hypercall arguments.

Accepting hypercall arguments which are either consistently in cached or
uncached is tricky and/or potentially slow, requiring a guest mapping lookup
to determine whether/when to do a cache clean or invalidate.

There are very few reasons, and no current use cases in practice, for a guest
to use uncached memory for their hypercall arguments. Therefore mandate that
all hypercall arguments must be mapped inner-cacheable.

Do not place any restriction on the outer-cacheability or on the cache
fill/flush strategy used.

If use cases arise then we can consider specific exemptions to this rule.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>