The CPSW CPDMA can process buffer descriptors placed as in internal
CPPI RAM as in DDR. This patch adds support in CPSW and CPDMA for
descs_pool_size mudule parameter, which defines total number of CPDMA CPPI
descriptors to be used for both ingress/egress packets processing:
 - memory size, required for CPDMA descriptor pool, is calculated basing
on number of descriptors specified by user in descs_pool_size and
CPDMA descriptor size and allocated from coherent memory (CMA area);
 - CPDMA descriptor pool will be allocated in DDR if pool memory size >
internal CPPI RAM or use internal CPPI RAM otherwise;
 - if descs_pool_size not specified in DT - the default value 256 will
be used which will allow to place CPDMA descriptors pool into the
internal CPPI RAM (current default behaviour);
 - CPDMA will ignore descs_pool_size if descs_pool_size = 0 for
backward comaptiobility with davinci_emac.

descs_pool_size is boot time setting and can't be changed once
CPSW/CPDMA is initialized.
Signed-off-by: NGrygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>