riscv: remove FIXMAP_PAGE_IO and fall back to its default value

If not defined in the arch, FIXMAP_PAGE_IO defaults to PAGE_KERNEL_IO,
which we defined when adding the svpbmt implementation.

So drop the FIXMAP_PAGE_IO riscv define.
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
Reviewed-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/20220511192921.2223629-11-heiko@sntech.deSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/include/asm/fixmap.h

@@ -45,8 +45,6 @@ enum fixed_addresses {
 	__end_of_fixed_addresses
 };
 
-#define FIXMAP_PAGE_IO		PAGE_KERNEL
-
 #define __early_set_fixmap	__set_fixmap
 
 #define __late_set_fixmap	__set_fixmap