gpio: at91: use dev_read_addr() to get base address

It is strange to use devfdt_get_addr_ptr(), then cast the pointer
back to uint32 because you could use devfdt_get_addr() without casting.

Convert it to dev_read_addr(), which is capable to CONFIG_OF_LIVE.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>