Docs: Remove out of sync struct members

'tfm_spm_partition_memory_data_t' no longer part of
'spm_partition_desc_t', remove it as it causes confusion
while searching symbols.

Change-Id: I685dc303d3c185f70e1a97a7a6f60074233c20b0
Signed-off-by: Ken Liu <Ken.Liu@arm.com>