drivers/of/base.c: Add of_property_read_u64_index

drivers/of/base.c: Add of_property_read_u64_index

BugLink: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1690412
There is of_property_read_u32_index but no u64 variant. This patch
adds one similar to the u32 version for u64.

Signed-off-by: Alistair Popple <alistair@popple.id.au>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
(cherry picked from commit 2475a2b6c877a0c8d1ca42c3f2b30f8ce518ac0b)
Signed-off-by: Breno Leitao <breno.leitao@gmail.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>