openrisc: add ioremap_nocache declaration before include asm-generic/io.h and sync ioremap prototype with it.

A future commit for the nds32 architecture bootstrap("asm-generic/io.h: move
ioremap_nocache/ioremap_uc/ioremap_wc/ioremap_wt out of ifndef CONFIG_MMU")
will move the ioremap_nocache out of the CONFIG_MMU ifdef. This means that
in order to suppress re-definition errors we need to setup #define's before
importing asm-generic/io.h.

Also, the change adds a prototype for ioremap where size is size_t so fix that
as well.
Signed-off-by: Greentime Hu <greentime@andestech.com>
Acked-by: Stafford Horne <shorne@gmail.com>