rockchip: make configure_emmc() empty for Firefly-RK3288

rockchip: make configure_emmc() empty for Firefly-RK3288

on v2016.03-rc3, size of SPL image compiled by gcc 5.3.0 is too large for
Firefly-RK3288. (it's fine for Rock2)

$ gcc --version
gcc (Ubuntu/Linaro 5.3.0-3ubuntu1~14.04) 5.3.0 20151204
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ ./tools/mkimage -n rk3288 -T rksd -d spl/u-boot-spl-dtb.bin u-boot-spl-dtb.img
Warning: SPL image is too large (size 0x80d0) and will not boot

to reduce size of SPL image, this patch makes configure_emmc() empty for
Firefly-RK3288 as same as Rock2.

Signed-off-by: FUKAUMI Naoki <naobsd@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-By: Vagrant Cascadian <vagrant@debian.org>