arm: realm: Add support for changing the state of memory

For a Realm, the guest physical address (in reality the IPA/GPA of the VM)
has an associated state (Realm IPA State, RIPAS) which is either of :
   RIPAS_RAM
   RIPAS_EMPTY

The state of the physical address decides certain behaviors. e.g., any access
to a RIPAS_EMPTY PA will generate a Synchronous External Abort back to the Realm,
from the RMM.

All "PA" that represents RAM for the Realm, must be set to RIPAS_RAM before
an access is made. When the initial image (e.g., test, DTB) of a Realm is
loaded, the hypervisor/VMM can transition the state of the loaded "area" to
RIPAS_RAM. The rest of the "RAM" must be transitioned by the test payload
before any access is made.

Similarly, a Realm could set an "IPA" to RIPAS_EMPTY, when it is about to use
the "unprotected" alias of the IPA. This is a hint for the host to reclaim the
page from the protected "IPA.

This patchs adds supporting helpers for setting the IPA state from Realm. These
will be used later for the Realm.

Co-developed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Joey Gouly <joey.gouly@arm.com>