Documentation: Remove CMA's dependency on architecture

CMA only depends on MMU. It doesn't depend on arch too much. such as ARM,
ARM64, X86, MIPS etc. so We remove the dependency of cma about the
architecture in kernel-parameters.txt.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/1600412758-60545-1-git-send-email-tiantao6@hisilicon.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>