spi: sh_qspi: Add DM support to SH QSPI driver

Add DM support to the SH QSPI driver while retaining non-DM support.
The later is required as this driver is used in SPL which has a size
limitation of 16 kiB.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
[jagan: use proper commit head]
Reviewed-by: Jagan Teki <jagan@openedev.com>