uboot-mediatek: add noncached_set_region prototype to fix build

uboot-mediatek: add noncached_set_region prototype to fix build

Building uboot-mediatek fails with GCC-14, uboot v2024.10 and
CONFIG_SYS_NONCACHED_MEMORY defined 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]

This is caused by upstream commit 7d6cee2cd0e2e2507aca1e3a6fe0e2cb241a116e
("cmd: cache: Remove weak functions") as this removes weak functions in
favor of arch-specific definitions.

This patch adds the function prototype for `noncached_set_region` to
arch-specific header for ARM. It also adds an include in cmd/cache.c to
make the function available there.

Fixes: #16697
Fixes: f8c22c9bff ("uboot-mediatek: update to U-Boot 2024.10")
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
[@dangowrt refreshed patch]
Signed-off-by: Daniel Golle <daniel@makrotopia.org>