MIPS: Document the cca= command-line parameter

Commit 35133692 ("[MIPS] Allow setting
of the cache attribute at run time") introduced the 'cca=' kernel
command-line parameter which allows overriding the kernel pages
cacheable attributes, document that parameter.

[ralf@linux-mips.org: replace @mips.com email addresses with it's imgtec.com
equivalent in this commit message.  Rephrase slightly for a bit more
pedantic correctness.]

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: blogic@openwrt.org
Cc: anemo@mba.ocn.ne.jp
Cc: chris.dearman@imgtec.com
Patchwork: https://patchwork.linux-mips.org/patch/7182/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>