arm64: head.S: document the use of callee saved registers

Now that the only remaining occurrences of the use of callee saved
registers are on the primary boot path, add a comment to the code
which register is used for what.

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>