doc: add optional rsaParent

Some implementation such as pcr-oracle prefers RSA 2048 to ECC NIST-P256
for the primary key. This commit introduces a new option, rsaParent, to
make it flexible to choose the assymetric algorithm for the primary key.

Signed-off-by: Gary Lin <glin@suse.com>
[jejb: update option to number 5]
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>