spi: cadence: Add read acess size switch

commit dec1153ce664081a90d55d4d5c3f110eb7927f92 from
git@git.assembla.com:cavium/WindRiver.linux.git

To improve performance add an 8-byte SDMA operation.
By default use 1-byte SDMA operations.

Signed-off-by: Witold Sadowski <wsadowski@marvell.com>
Change-Id: I09f2b02bc886a7dfd22842bf553536290c0b9c24
Reviewed-on: https://sj1git1.cavium.com/c/IP/SW/kernel/linux/+/67408
Reviewed-by: Chandrakala Chavva <cchavva@marvell.com>
Tested-by: Sunil Kovvuri Goutham <sgoutham@marvell.com>
Signed-off-by: Ruiqiang Hao <Ruiqiang.Hao@windriver.com>