fw_base.S: Fix comment errors

When calling '_reset_regs', it'll reset all registers except some
specific registers (ra, a0, a1, and a2).

Both boot HART and non-boot HARTs will execute the '_start_warm'
function. Therefore, when '_reset_regs' is called in '_start_warm', it
will reset all registers except some specific registers (ra, a0, a1 and
a2) for both boot HART and non-boot HARTs.

Signed-off-by: Zhang Runmin <fmrt19zrmin@163.com>
Reviewed-by: Anup Patel <anup@brainfault.org>