configs: Rename roc-rk3399-pc -> roc-pc-rk3399 defconfig

roc-rk3399-pc_defconfig is committed in below

commit <8a681f4c> ("rockchip: rk3399:
Add ROC-RK3399-PC support")

which doesn't follow the existing defconfigs on rk3399.

So, rename as followed with other rk3399 defconfigs.

Cc: Levin Du <djw@t-chip.com.cn>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>