ARM: imx3: Remove imx3 soc_init()

imx31_soc_init() and imx35_soc_init() were only used by i.MX31/35 board
files. Since the board files users are gone, it is safe to remove
imx3 soc_init() functions.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>