arm64: head: fix code comments in set_cpu_boot_mode_flag

Up to here, the CPU boot mode can either be EL1 or EL2.
Correct the code comments a bit.

Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20210518101405.1048860-5-aisheng.dong@nxp.com
Signed-off-by: Will Deacon <will@kernel.org>