tools/litex_json2dts_zephyr: add mapped SPI flash for XIP support

This allows to execute a firmware loaded in the SPI flash.

No support for partitions yet: this does not yet support cohabitating the
bitstream and firmware yet, so the bitstream has to be loaded to CRAM
directly for now (default for --load).

This allows loading larger Zephyr examples, and was required to fit
the shell on the Crosslink-NX board.

See litex-hub/zephyr-on-litex-vexriscv#13

litex/tools/litex_json2dts_zephyr.py

@@ -207,11 +207,6 @@ def peripheral_handler(name, parm, csr):
         'alias': 'eth0',
         'config_entry': 'ETH_LITEETH'
     },
-    'spiflash': {
-        'handler': peripheral_handler,
-        'alias': 'spi0',
-        'config_entry': 'SPI_LITESPI'
-    },
     'sdcard_block2mem': {
         'handler': peripheral_handler,
         'alias': 'sdcard_block2mem',
@@ -263,6 +258,11 @@ def peripheral_handler(name, parm, csr):
         'handler': ram_handler,
         'alias': 'ram0',
     },
+    'spiflash': {
+        'handler': ram_handler,
+        'alias': 'flash0',
+        'config_entry': 'XIP'
+    },
     'identifier_mem': {
         'handler': peripheral_handler,
         'alias': 'dna0',