riscv: Allow ZONE_DMA32 to be disabled

Blackhole has no memory below 4 GB so modify arch/riscv Kconfig to allow
ZONE_DMA32 to be disabled.

The following oops occurs when ZONE_DMA32 is enabled. The boot does
complete and the system does work okay. However, the oops adds noise
and may unnecessarily alarm users.

[    0.017757] [<ffffffff80014f34>] dump_backtrace+0x1c/0x24
[    0.017771] [<ffffffff80002310>] show_stack+0x28/0x34
[    0.017777] [<ffffffff8000f21a>] dump_stack_lvl+0x48/0x66
[    0.017783] [<ffffffff8000f24c>] dump_stack+0x14/0x1c
[    0.017788] [<ffffffff801b4428>] warn_alloc+0xc4/0x144
[    0.017798] [<ffffffff801b4a7a>] __alloc_frozen_pages_noprof+0x5d2/0xb78
[    0.017805] [<ffffffff801b502e>] __alloc_pages_noprof+0xe/0x22
[    0.017811] [<ffffffff80095d20>] atomic_pool_expand+0x84/0x1b2
[    0.017819] [<ffffffff80a0ea24>] __dma_atomic_pool_init+0x46/0x9e
[    0.017827] [<ffffffff80a0eb18>] dma_atomic_pool_init+0x9c/0x122
[    0.017833] [<ffffffff800100ec>] do_one_initcall+0x38/0x174
[    0.017839] [<ffffffff80a0111a>] kernel_init_freeable+0x206/0x272
[    0.017847] [<ffffffff809a4d58>] kernel_init+0x1e/0x13a
[    0.017854] [<ffffffff809ae03e>] ret_from_fork+0xe/0x18
[    0.017860] Mem-Info:
[    0.101848] active_anon:0 inactive_anon:0 isolated_anon:0
[    0.101848]  active_file:0 inactive_file:0 isolated_file:0
[    0.101848]  unevictable:0 dirty:0 writeback:0
[    0.101848]  slab_reclaimable:38 slab_unreclaimable:266
[    0.101848]  mapped:0 shmem:0 pagetables:65
[    0.101848]  sec_pagetables:0 bounce:0
[    0.101848]  kernel_misc_reclaimable:0
[    0.101848]  free:712211 free_pcp:0 free_cma:0
[    0.141069] 0 total pagecache pages
[    0.144668] 0 pages in swap cache
[    0.148139] Free swap  = 0kB
[    0.151091] Total swap = 0kB
[    0.154044] 1048576 pages RAM
[    0.157104] 0 pages HighMem/MovableOnly
[    0.161043] 335051 pages reserved
[    0.164473] DMA: failed to allocate 348 KiB GFP_KERNEL|GFP_DMA32 pool for atomic allocation

Signed-off-by: Drew Fustini <dfustini@tenstorrent.com>

Author: tt-fustini

arch/riscv/Kconfig

@@ -227,7 +227,6 @@ config RISCV
 	select UACCESS_MEMCPY if !MMU
 	select VDSO_GETRANDOM if HAVE_GENERIC_VDSO && 64BIT
 	select USER_STACKTRACE_SUPPORT
-	select ZONE_DMA32 if 64BIT
 
 config RUSTC_SUPPORTS_RISCV
 	def_bool y