dt-bindings: devfreq: rk3399_dmc: fix clocks in example

dt-bindings: devfreq: rk3399_dmc: fix clocks in example

The clocks in the example use 'SCLK_DDRCLK', which does not exist in
the rk3399-cru.h, correct it to 'SCLK_DDRC'.

Ref: commit b79377ecc602 ("clk: rockchip: add SCLK_DDRC id for rk3399 ddrc")
Fixes: df29b5b3b99b ("Documentation: bindings: add dt documentation for rk3399 dmc")
Signed-off-by: Chukun Pan <amadeus@jmu.edu.cn>
Link: https://lore.kernel.org/r/20211001150024.18672-1-amadeus@jmu.edu.cn
Signed-off-by: Rob Herring <robh@kernel.org>