riscv: use the generic ioremap code

Use the generic ioremap code instead of providing a local version.
Note that this relies on the asm-generic no-op definition of
pgprot_noncached.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Paul Walmsley <paul.walmsley@sifive.com>
Tested-by: Paul Walmsley <paul.walmsley@sifive.com> # rv32, rv64 boot
Acked-by: Paul Walmsley <paul.walmsley@sifive.com> # arch/riscv