unicore32: replace unicore32-specific iomap functions with generic lib implementation

1. define and enable CONFIG_GENERIC_IOMAP
2. define unicore32-specific PCI_IOBASE for asm-generic/io.h
3. define HAVE_ARCH_PIO_SIZE and unicore32-specific PIO_* macros
4. remove all unicore32-specific iomap functions

Signed-off-by: Guan Xuetao <gxt@mprc.pku.edu.cn>
Reviewed-and-Acked-by: Arnd Bergmann <arnd@arndb.de>