arm64: add comments about EC exception levels

To make it clear which exceptions can't be taken to EL1 or EL2, add
comments next to the ESR_ELx_EC_* macro definitions.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Kristina Martsenko <kristina.martsenko@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>