powerpc: introduce and document sdhci,wp-inverted property for eSDHC

eSDHC block in MPC837x SOCs reports inverted write-protect state, soon
sdhci-of driver will look for sdhci,wp-inverted properties to decide
whether apply a specific quirk.

So, document the property and add it to device tree source files.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Cc: Pierre Ossman <pierre@ossman.eu>
Cc: Kumar Gala <galak@kernel.crashing.org>
Cc: David Vrabel <david.vrabel@csr.com>
Cc: Ben Dooks <ben@fluff.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: <linux-mmc@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>