ARM: ux500: DT: Rename root node "soc-u9500" into "soc"

This root node is used in the whole family of chips, the a9500
is just one particular instance. Nodes should be named after
the type of object rather than identity.

Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@stericsson.com>
Reviewed-by: Philippe Langlais <philippe.langlais@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>