spi: sifive: Add spi-mem exec op

SiFive SPI controller is responsible to handle the
slave devices like mmc spi and spi nor flash.

The controller is designed such a way that it would
handle the slave transactions based on the I/O protocol
numbers, example if spi nor slave send quad write opcode
it has to send alone with I/O protocol number of 4 and
if it try to send data it has to send I/O protocol number
along with 4 line data.

But the current spi-xfer code from spi-mem is combining
the opcode and address in a single transaction, so the
SPI controller will be unable to identify the I/O protocol
number of opcode vs address.

So, add the spi-mem exec_op with spi-xfer of opcode, address
and data as a separate transaction. This doesn't remove
the .xfer of dm_spi_ops since mmc spi will make use of it.

Note: This code might have moved to the spi-mem core area
once we have done the dedicated tests on other controllers
and have real reason to move.

Cc: Vignesh R <vigneshr@ti.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Tested-by: Sagar Kadam <sagar.kadam@sifive.com>