[ARM] pxa: use __raw_writel()/__raw_readl() for ssp_xxxx()

1. change SSP register definitions from absolute virtual addresses to
   offsets

2. use __raw_writel()/__raw_readl() for functions of ssp_xxxx()

Signed-off-by: eric miao <eric.miao@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>