generic: add missing symbol ARM64_CONTPTE

Add the missing symbol to fix build halt on aarch64 targets:
```
Contiguous PTE mappings for user memory (ARM64_CONTPTE) [Y/n/?] (NEW) make[7]: *** [scripts/kconfig/Makefile:85: syncconfig] Error 1
make[6]: *** [Makefile:686: syncconfig] Error 2
```

Signed-off-by: Shiji Yang <[email protected]>
Link: openwrt/openwrt#20954
Signed-off-by: Christian Marangi <[email protected]>

Author: DragonBluep

target/linux/generic/config-6.12

@@ -339,6 +339,7 @@ CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MIN=8
 # CONFIG_ARM64_AMU_EXTN is not set
 # CONFIG_ARM64_BTI is not set
 CONFIG_ARM64_CNP=y
+# CONFIG_ARM64_CONTPTE is not set
 # CONFIG_ARM64_E0PD is not set
 # CONFIG_ARM64_EPAN is not set
 # CONFIG_ARM64_ERRATUM_1024718 is not set