vme: fake: fix build for 64-bit dma_addr_t

casting between dma_addr_t and a pointer is generally tricky,
as they might not be the same size and almost never point into
the same address space. With 32-bit ARM systems and LPAE, we
get this warning for the vme_fake driver that stores a pointer
in a dma_addr_t variable:

drivers/vme/bridges/vme_fake.c: In function 'fake_slave_set':
drivers/vme/bridges/vme_fake.c:204:29: error: assignment makes pointer from integer without a cast [-Werror=int-conversion]

To make this clearer while fixing the warning, I'm adding
a set of helper functions for the type conversion.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Martyn Welch <martyn@welchs.me.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>