cpu_func.h: provide noncached_set_region prototype to fix build

Due to the removal of weak functions in 7d6cee2cd0 ("cmd: cache: Remove
weak function"), U-Boot fails to compile after updating to v2024.10 for
mediatek target in OpenWrt with GCC-14 with error:
cmd/cache.c: In function 'do_dcache':
cmd/cache.c:57:25: error: implicit declaration of function
	'noncached_set_region' [-Wimplicit-function-declaration]

Thus, provide a prototype in include/cpu_func.h to fix a build
error in cmd/cache.c, since related prototypes are also located there.

The issue occurred after the update of uboot-mediatek in OpenWrt to
v2024.10, in combination with GCC-14 toolchain. It was reported and
discussed in https://github.com/openwrt/openwrt/issues/16697, and
temporarily fixed with
92ca322dd1.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/issues/16697
Link: 92ca322dd1
Fixes: 7d6cee2cd0 ("cmd: cache: Remove weak function")
Signed-off-by: Tom Rini <trini@konsulko.com>

include/cpu_func.h

@@ -80,6 +80,7 @@ void invalidate_icache_all(void);
  * Return: 0 if OK
  */
 int noncached_init(void);
+void noncached_set_region(void);
 
 phys_addr_t noncached_alloc(size_t size, size_t align);