ArmVirtPkg: ArmCcaLib: Cache current world value

IsRealm() probes to check if the code is executing
in a Realm context by checking if RME is supported
and then issuing a RSI_VERSION command to check it
is supported.

Instead of calling RSI_VERSION command every time the
IsRealm() is called, cache the world value we are
running in, to return the value in subsequent calls.

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>