mtd: denali: fix warning when compiled for 64bit system

The 64-bit compiler (ex. aarch64) emits "warning: cast from pointer
to integer of different size".

Make it work with 64bit DMA address while I am here.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Tom Rini <trini@konsulko.com>