dmaengine: ste_dma40: use writel_relaxed for lcxa

lcpa and lcla are written often and the cache_sync() overhead in writel
is costly, especially for wlan where every single network packet (in RX
mode) corresponds to a separate DMA transfer.

Signed-off-by: Per Forlin <per.forlin@stericsson.com>
Reviewed-by: Narayanan Gopalakrishnan <narayanan.gopalakrishnan@stericsson.com>
Reviewed-by: Rabin Vincent <rabin.vincent@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>