ARM PAC FAQ fix: Read HCR_EL2 instead of clobbering with SCTLR_EL1

[droe]

The sample code for setting up PAC is missing the read register call for HCR_EL2. This leads to clobbering HCR_EL2 with whatever the bits in SCTLR_EL1 were.

Fix by adding the missing register read call.