arm64: RME: Hide KVM_CAP_READONLY_MEM for realm guests

For protected memory read only isn't supported by the RMM. While it may
be possible to support read only for unprotected memory, this isn't
supported at the present time.

Note that this does mean that ROM (or flash) data cannot be emulated
correctly by the VMM as the stage 2 mappings are either always
read/write or are trapped as MMIO (so don't support operations where the
syndrome information doesn't allow emulation, e.g. load/store pair).

This restriction can be lifted in the future by allowing the unprotected
stage 2 mappings to be made read only.

Signed-off-by: Steven Price <steven.price@arm.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
---
Changes since v7:
 * Updated commit message to spell out the impact on ROM/flash
   emulation.