test: add discovery of el3 spmd logical partitions

This patch adds a test to query the presence of an SPMD logical
partition and also tests basic functionality using the
ffa_partition_info_get_regs abi. Note that the register based discovery
reports the presence of el3 spmd logical partitions whereas the memory
based discovery interface does not report the el3 spmd logical
partitions. To that end, the patch adds helper functions to use the
register based discovery, and also refactors code that can be shared
between the register and memory based interfaces.

Signed-off-by: Raghu Krishnamurthy <raghu.ncstate@gmail.com>
Change-Id: I755ffe4098c635de2c6aeb0ebe73eb16c3acd206
4 files changed