arm64: realm: Double the IPA space

The Realm's IPA space is divided into 2 halves. Protected
(lower half) and Unprotected (upper half). KVM implements
aliasing of the IPA, where the unprotected IPA is alias of
the corresponding protected ipa. Thus we must double the
IPA space required for a given VM.

Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>