dma: mv_xor: Fix mis-usage of mmio 'base' and 'high_base' registers

Despite requesting two memory resources, called 'base' and 'high_base', the
driver uses explicitly only the former. The latter is being used implicitly
by addressing at offset +0x200, which in practice accesses high_base.

In other words, the current driver breaks if the second memory resource
is ever place at an offset different from +0x200.

This patch fixes the above by defining the registers with the offset from
high_base, and use high_base explicitly where appropriate.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>