MIPS: select ARCH_KEEP_MEMBLOCK unconditionally

While removing allnoconfig_y from Kconfig, ARCH=mips allnoconfig builds
started failing with the error:

WARNING: modpost: vmlinux.o(.text+0x9c70): Section mismatch in reference
from the function reserve_exception_space() to the function
.meminit.text:memblock_reserve()
The function reserve_exception_space() references the function __meminit
memblock_reserve().
This is often because reserve_exception_space lacks a __meminit
annotation or the annotation of memblock_reserve is wrong.
ERROR: modpost: Section mismatches detected.
Set CONFIG_SECTION_MISMATCH_WARN_ONLY=y to allow them.

allnoconfig disables DEBUG_KERNEL and thus ARCH_KEEP_MEMBLOCK, which
changes __init_memblock to be equivalent to __meminit triggering the
above error.

Link: https://lore.kernel.org/linux-kbuild/[e-mail]/
Reported-by: Guenter Roeck <[e-mail]>
Signed-off-by: Nick Desaulniers <[e-mail]>
Reviewed-by: Masahiro Yamada <[e-mail]>
Signed-off-by: Thomas Bogendoerfer <[e-mail]>

arch/mips/Kconfig

···
 	select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST
 	select ARCH_HAS_UBSAN_SANITIZE_ALL
 	select ARCH_HAS_GCOV_PROFILE_ALL
-	select ARCH_KEEP_MEMBLOCK if DEBUG_KERNEL
+	select ARCH_KEEP_MEMBLOCK
 	select ARCH_SUPPORTS_UPROBES
 	select ARCH_USE_BUILTIN_BSWAP
 	select ARCH_USE_CMPXCHG_LOCKREF if 64BIT