arch/hexagon: fix kernel/dma.c build warning

arch/hexagon: fix kernel/dma.c build warning

Fix build warning in arch/hexagon/kernel/dma.c by casting a void *
to unsigned long to match the function parameter type.

../arch/hexagon/kernel/dma.c: In function 'arch_dma_alloc':
../arch/hexagon/kernel/dma.c:51:5: warning: passing argument 2 of 'gen_pool_add' makes integer from pointer without a cast [enabled by default]
../include/linux/genalloc.h:112:19: note: expected 'long unsigned int' but argument is of type 'void *'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Rich Felker <dalias@libc.org>
Cc: linux-sh@vger.kernel.org
Patch-mainline: linux-kernel @ 07/20/2018, 20:17
[rkuo@codeaurora.org: fixed architecture name]
Signed-off-by: Richard Kuo <rkuo@codeaurora.org>