ARM: dts: rockchip: fix rk3188 emmc pull references

Fix a copy'n'paste error making the rk3188 emmc pinctrl nodes reference
the pcfg_pull_default setting that is not available on rk3188.

Reported-by: Naoki FUKAUMI <naobsd@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>