[ARM] dma-mapping: improve type-safeness of DMA translations

OMAP at least gets the return type(s) for the DMA translation functions
wrong, which can lead to subtle errors.  Avoid this by moving the DMA
translation functions to asm/dma-mapping.h, and converting them to
inline functions.

Fix the OMAP DMA translation macros to use the correct argument and
result types.

Also, remove the unnecessary casts in dmabounce.c.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>