lib/genalloc.c: add support for specifying the physical address

So we can specify the virtual address as the base of the pool chunk and
then get physical addresses for hardware IP.

For example on at91 we will use this on spi, uart or macb

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Patrice VILCHEZ <patrice.vilchez@atmel.com>
Cc: Jes Sorensen <jes@wildopensource.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>