ARM: pxa3xx: provide specific platform_devices for all ssp ports

Currently, devices for SSP ports 1, 2 and 3 are registered as compatible
devices to pxa27x-ssp. While the actual IP core is comparable, there are
some subtle differences which users of the SSP ports address by looking at
the 'type' field.

By registering devices of type 'pxa27x-ssp', this 'type' field is
incorrectly set to PXA27x_SSP which confuses the users.

To fix this, provide specific ssp port plaform devices which use
'pxa3xx-ssp' as driver name, an instantiate them from pxa3xx.c.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>