dmaengine: ste_dma40: Calculate number of logical channels from physical ones

This change will cost ~25KB of memory, but it's worth the trade-off,
as it removes a great deal of overhead. It means that instead of only
allocating memory for the logical channels in use, it does so for all
available ones, which is 32 per physical channel. However, this now
means we can remove some platform data and we don't have to worry
about adding vendor specific variables to Device Tree.

Acked-by: Vinod Koul <vnod.koul@intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>