riscv: mm: mark noncoherent_supported as __ro_after_init

The noncoherent_supported indicates whether the HW is coherent or not,
it won't change after booting, mark it as __ro_after_init.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230614165504.532-4-jszhang@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: xhackerustc

arch/riscv/mm/dma-noncoherent.c

@@ -10,7 +10,7 @@
 #include <linux/mm.h>
 #include <asm/cacheflush.h>
 
-static bool noncoherent_supported;
+static bool noncoherent_supported __ro_after_init;
 
 void arch_sync_dma_for_device(phys_addr_t paddr, size_t size,
 			      enum dma_data_direction dir)