fix(thinknode_m9): define SPI_FREQUENCY for the non-MUI build (#11546)

The M9's variant.h defines ST7789_CS, so TFTDisplay.cpp compiles its
ST7789 LGFX branch, which reads SPI_FREQUENCY for the panel write clock
(SPI_READ_FREQUENCY, its pair, is already in variant.h). The flag was only
set in the -tft env, so `build (thinknode_m9, esp32s3)` has failed on
develop since the board landed in #10908:

  src/graphics/TFTDisplay.cpp:504:30: error: 'SPI_FREQUENCY' was not
  declared in this scope; did you mean 'SD_SPI_FREQUENCY'?

Move the flag up into thinknode_m9_base, keeping the 75 MHz the -tft env
already used for the same panel and matching the SD card's 75 MHz on the
bus they share. The -tft env inherits the base flags, so device-ui's
LGFX_GENERIC.h - which falls back to 20 MHz when the macro is absent -
still sees the identical value.

variants/esp32s3/ELECROW-ThinkNode-M9/platformio.ini

@@ -31,6 +31,7 @@ build_flags =
   -D SDCARD_INIT_SPI
   -D SD_SPI_FREQUENCY=75000000U
   -D HAS_SCREEN=1
+  -D SPI_FREQUENCY=75000000
 ;  -D COMPASS_SENSOR_DEBUG=1
 lib_deps = ${esp32s3_base.lib_deps}
   # renovate: datasource=custom depName=LovyanGFX packageName=lovyan03/library/LovyanGFX
@@ -82,7 +83,6 @@ build_flags =
   -D LGFX_SCREEN_WIDTH=240
   -D LGFX_SCREEN_HEIGHT=320
   -D LGFX_INVERT_LIGHT=true
-  -D SPI_FREQUENCY=75000000
 ;  -D MAP_FULL_REDRAW
   -D MUI_WIFI_PS_MIN_MODEM
   -D DEFAULT_FTP_SERVER_NETWORK_TYPE_ESP32=NETWORK_ESP32