Cleanup the comment for m68k linux boot argument passing.

This patch clarifies the way m68k passes linux boot argument.
The one gotcha here is that the assembly instruction that
the compiler uses to jump to the kernel is 'jsr' which pushes the
program counter for the instruction after the jsr into the stack pointer.

Signed-off-by: Richard Retanubun <RichardRetanubun@RuggedCom.com>
Signed-off-by: Wolfgang Denk <wd@denx.de>