ARM: dts: Disable smc91x on n900 until bootloader dependency is removed

I added smc91x support but turns out we currently do not set the
smc91x timings in gpmc.c but rely on the bootloader timings. This
produces the following error unless the smc91x GPMC timings are
initialized by the bootloader:

Unhandled fault: external abort on non-linefetch (0x1008) at 0xd080630e
...
[<c04067fc>] (smc_drv_probe) from [<c038e9c4>] (platform_drv_probe+0x2c/0x5c)
[<c038e9c4>] (platform_drv_probe) from [<c038d450>] (driver_probe_device+0x104/0x22c)
[<c038d450>] (driver_probe_device) from [<c038d60c>] (__driver_attach+0x94/0x98)
[<c038d60c>] (__driver_attach) from [<c038bc3c>] (bus_for_each_dev+0x54/0x88)
[<c038bc3c>] (bus_for_each_dev) from [<c038cc3c>] (bus_add_driver+0xd8/0x1d8)
[<c038cc3c>] (bus_add_driver) from [<c038dd74>] (driver_register+0x78/0xf4)
[<c038dd74>] (driver_register) from [<c0008924>] (do_one_initcall+0x80/0x1c0)
[<c0008924>] (do_one_initcall) from [<c0852d9c>] (kernel_init_freeable+0x1b8/0x28c)
[<c0852d9c>] (kernel_init_freeable) from [<c05ce86c>] (kernel_init+0x8/0xec)
[<c05ce86c>] (kernel_init) from [<c000e728>] (ret_from_fork+0x14/0x2c)

Let's fix the issue by disabling the smc91x module for now until we
have sorted out the issues in gpmc.c.

Reported-by: Kevin Hilman <khilman@linaro.org>
Tested-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>