serial: sifive: Cast dev_read_addr() with uintptr_t

dev_read_addr() returns fdt_addr_t which is now a 64-bit address.
In a 32-bit build, this causes the following warning seen when
building serial_sifive.c:

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

Cast the return value with uintptr_t.

Signed-off-by: Bin Meng <bin.meng@windriver.com>