spi: Add SiFive SPI driver

This patch adds SiFive SPI driver. The driver is 100% DM driver
and it determines input clock using clk framework.

The SiFive SPI block is found on SiFive FU540 SOC and is used to
access flash and MMC devices on SiFive Unleashed board.

This driver implementation is inspired from the Linux SiFive SPI
driver available in Linux-5.2 or higher and SiFive FSBL sources.

Signed-off-by: Bhargav Shah <bhargavshah1988@gmail.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>