sizeof(dma_addr_t) == 8 shouldn't introduce any functional change, because
we can't have DMA on sandbox. For now it suppresses benign warnings about
mismatched pointer and integer sizes when some headers are included.

sizeof(phys_addr_t) == 8 was already the case on systems with
__x86_64__. As CONFIG_64BIT now is set according to the bitness
of the compiler, we can migrate this fully to Kconfig.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>