ARM: Remove PicoXcell platform support

PicoXcell has had nothing but treewide cleanups for at least the last 8
years and no signs of activity. The most recent activity is a yocto vendor
kernel based on v3.0 in 2015.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Jamie Iles <jamie@jamieiles.com>
Cc: Russell King <linux@armlinux.org.uk>
Link: https://lore.kernel.org/r/20201210200315.2965567-3-robh@kernel.org

'
Signed-off-by: Arnd Bergmann <arnd@arndb.de>