asm-generic/io.h: convert readX defines to functions

E.g. readl is defined like this

 #define readl(addr) __le32_to_cpu(__raw_readl(addr))

If a there is a readl() call that doesn't check the return value
this will cause a compile warning on big endian machines due to
the __le32_to_cpu macro magic.

E.g. code like this:

	readl(addr);

will generate the following compile warning:

warning: value computed is not used [-Wunused-value]

With this patch we get rid of dozens of compile warnings on s390.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>