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>

Documentation/kernel-parameters.txt

@@ -566,6 +566,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			possible to determine what the correct size should be.
 			This option provides an override for these situations.
 
+	cca=		[MIPS] Override the kernel pages' cache coherency
+			algorithm.  Accepted values range from 0 to 7
+			inclusive. See arch/mips/include/asm/pgtable-bits.h
+			for platform specific values (SB1, Loongson3 and
+			others).
+
 	ccw_timeout_log [S390]
 			See Documentation/s390/CommonIO for details.