ENGR00273432 DMA: imx-sdma: Allocate 4KB iram page size for bd

ENGR00273432 DMA: imx-sdma: Allocate 4KB iram page size for bd

One bd actually needs 4KB page size. So changed the iram_alloc()
size parameter to allocate a precise iram memory for bd.

This patch also removed the extra iram pool for SDMA due to its
insufficient total size: SDMA allows around 48 channels to work
simultaneously, so it's better to allocate memory from iram pool
directly.

[There will be 3KB size waste in sdma->channel_control, which is
640B but actually using 4KB due to 4KB alignment for iram pool.]

Acked-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Nicolin Chen <b42378@freescale.com>