asm-generic: don't provide __ioremap

__ioremap is not a kernel API, but used for helpers with differing
semantics in arch code.  We should not provide it in as-generic.

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
Signed-off-by: Arnd Bergmann <arnd@arndb.de>