soc/cores/spi/spi_mmap: add dts support

spi_mmap will require target integration, for example:

    self.spi_mmap_1 = SPIMMAP(...)
    self.add_core_dts("spi_mmap_1", self.spi_mmap_1.dts_compatible)

litex/soc/cores/spi/spi_mmap.py

@@ -642,6 +642,8 @@ def __init__(self, pads, ctrl, data_width, sys_clk_freq):
 # SPIMMAP ------------------------------------------------------------------------------------------
 
 class SPIMMAP(LiteXModule):
+    dts_compatible = "litex,spi_mmap"
+
     def __init__(self, pads, data_width, sys_clk_freq,
         tx_origin = 0x0000_0000,
         rx_origin = 0x0000_0000,