[ARM] pxa: remove incorrect select PXA_SSP in Kconfig

PXA_SSP is actually used by drivers like drivers/spi/pxa2xx_spi.c and
sound/soc/pxa/pxa-ssp.c, not by boards. Remove those incorrect 'select'
from Kconfig and make SOC_PXA_SSP to select.

Signed-off-by: Eric Miao <eric.y.miao@gmail.com>