ARM: smccc: Update HVC comment to describe new quirk parameter

Commit 680a0873

 ("arm: kernel: Add SMC structure parameter") added
a new "quirk" parameter to the SMC and HVC SMCCC backends, but only
updated the comment for the SMC version. This patch adds the new
paramater to the comment describing the HVC version too.

Signed-off-by: Will Deacon <will.deacon@arm.com>