ARM: dts: rockchip: Fix rk3288-rock2 vcc_flash name

There is no functional change from this, but it is confusing to find two
copies of vcc_sys and no vcc_flash when looking in
/sys/class/regulator/*/name.

Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>