Update doc links in the porting guide
Update doc links in the porting guide

GIC v2 and v3 specification references in the porting guide
should refer to publically visible links, not ARM internal links.

Change-Id: Ib47c8adda6a03581f23bcaed72d71c08c7dd9fb1
Signed-off-by: Yuping Luo <c934c18f7091741ac6b3251fd2520083eefb247d@arm.com>

lsigithub/arm-trusted-firmware_public,sandrine-bailleux-arm/arm-trusted-firmware,achingupta/arm-trusted-firmware,sbranden/arm-trusted-firmware,achingupta/arm-trusted-firmware,sandrine-bailleux-arm/arm-trusted-firmware,sbranden/arm-trusted-firmware,sandrine-bailleux/arm-trusted-firmware,sandrine-bailleux/arm-trusted-firmware,davwan01/arm-trusted-firmware,davwan01/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public