spi: cadence: Add polling mode support

commit 3a8e291b73e039742bc3494092c82eaf6819f112 from
git@git.assembla.com:cavium/WindRiver.linux.git

If IRQ is not configured, switch driver to polling
mode instead of returning with error.
Code is using SDMA interrupt flag to determine
SDMA operation status, and stig ready flag to
determine stig engine readiness.

Signed-off-by: Witold Sadowski <wsadowski@marvell.com>
Change-Id: Ife85c1f24294c11c62b8f5ec0af5d56b65a8ff96
Reviewed-on: https://sj1git1.cavium.com/c/IP/SW/kernel/linux/+/67407
Tested-by: sa_ip-sw-jenkins <sa_ip-sw-jenkins@marvell.com>
Reviewed-by: Chandrakala Chavva <cchavva@marvell.com>
Reviewed-by: Sunil Kovvuri Goutham <sgoutham@marvell.com>
Signed-off-by: Ruiqiang Hao <Ruiqiang.Hao@windriver.com>