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 <[email protected]>
Cc: Chin Liang See <[email protected]>
Cc: Dinh Nguyen <[email protected]>
Cc: Pavel Machek <[email protected]>
Cc: Stefan Roese <[email protected]>
Cc: Vince Bridgers <[email protected]>
Reviewed-by: Stefan Roese <[email protected]>
Acked-by: Pavel Machek <[email protected]>
Reviewed-by: Jagannadha Sutradharudu Teki <[email protected]>