gpio: sifive: Fix -Wint-to-pointer-cast warning

gpio: sifive: Fix -Wint-to-pointer-cast warning

dev_read_addr() returns a value of type fdt_addr_t which is a 64-bit
address and plat->base is a pointer. In a 32-bit build, this causes the
following warning seen when building sifive-gpio.c:

  warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]

Change to use dev_read_addr_ptr().

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>