soc/mediatek/mtk_adsp: Support __nocache

This is a pre-existing (but rarely used) Zephyr linker tag.  Support
it instead of reinventing something similar.

Signed-off-by: Andy Ross <andyross@google.com>

soc/mediatek/mt8xxx/Kconfig.defconfig

@@ -31,6 +31,9 @@ config IRQ_OFFLOAD_NESTED
 	default n if SOC_SERIES_MT818X
 	default y
 
+config NOCACHE_MEMORY
+	default y
+
 config MTK_ADSP_TIMER
 	default y
 config XTENSA_TIMER

soc/mediatek/mt8xxx/linker.ld

@@ -110,6 +110,11 @@ SECTIONS {
   _end = .;
   _mtk_adsp_dram_end = .;
 
+  .nocache (NOLOAD) : {
+    . = ALIGN(4096);
+    *(.nocache .nocache.*)
+  } > dram
+
   /* Non-runtime-loaded sections below */
 
 #include <zephyr/linker/debug-sections.ld>