asm-generic/io.h: suppress endianness warnings for relaxed accessors

Copy the forced type casts from the normal MMIO accessors to suppress
the sparse warnings that point out __raw_readl() returns a native endian
word (just like readl()).

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>