ARM: at91: at91_dt_defconfig: set MMC_SPI to module

Change CONFIG_MMC_SPI from built-in to module in the
at91_dt_defconfig. This allows the MMC over SPI driver to
be loaded only when needed for WILC subsystem.

Signed-off-by: Manikandan Muralidharan <manikandan.m@microchip.com>
Link: https://lore.kernel.org/r/20250923094552.430595-1-manikandan.m@microchip.com
Signed-off-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>

Author: manikandan-m11

arch/arm/configs/at91_dt_defconfig

@@ -181,7 +181,7 @@ CONFIG_MMC_SDHCI=y
 CONFIG_MMC_SDHCI_PLTFM=y
 CONFIG_MMC_SDHCI_OF_AT91=y
 CONFIG_MMC_ATMELMCI=y
-CONFIG_MMC_SPI=y
+CONFIG_MMC_SPI=m
 CONFIG_NEW_LEDS=y
 CONFIG_LEDS_CLASS=y
 CONFIG_LEDS_GPIO=y