hwrng: picoxcell - remove unbuildable picoxcell TRNG

The driver for the "Picochip picoXcell true random number generator" was
added in v2.6.39. Its Kconfig symbol has always depended on
PICOXCELL_PC3X3. But that Kconfig symbol has never been part of the
tree. This means this driver has never been buildable. Let's remove it.
It can be re-added if its dependencies are actually part of the tree.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Jamie Iles <jamie@jamieiles.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>