dt: socfpga: Rename snps, dw-spi-mmio to snps, dw-apb-ssi

dt: socfpga: Rename snps, dw-spi-mmio to snps, dw-apb-ssi

Linux now also contains SPI driver, yet the name is 'snps,dw-apb-ssi'.
Fix the naming before we have to support both names.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <clsee@opensource.altera.com>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Pavel Machek <pavel@denx.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Vince Bridgers <vbridger@opensource.altera.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Acked-by: Pavel Machek <pavel@denx.de>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>