ARM: pxa: Remove unused clock_enable field from struct pxa2xx_spi_master

Use for struct pxa2xx_spi_master clock_enable field was removed years ago
from the pxa2xx-spi driver by the commit 2f1a74e5 ("[ARM] pxa: make
pxa2xx_spi driver use ssp_request()/ssp_free()").

Therefore remove it from structure definition, documentation and from
couple affected board files.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>