rockchip: rk3188: add README.rockchip paragraph describing sd boot

Building sd images for rk3188 requires more steps due to the needed split
into TPL and SPL as loaders. Describe how to build an image for it in a
separate paragraph in the READER.rockchip file.
Signed-off-by: NHeiko Stuebner <heiko@sntech.de>
Reviewed-by: NKever Yang <kever.yang@rock-chips.com>

doc/README.rockchip

@@ -147,6 +147,32 @@ For evb_rk3036 board:
 Note: rk3036 SDMMC and debug uart use the same iomux, so if you boot from SD, the
       debug uart must be disabled
 
+
+Booting from an SD card on RK3188
+=================================
+
+For rk3188 boards the general storage onto the card stays the same as
+described above, but the image creation needs a bit more care.
+
+The bootrom of rk3188 expects to find a small 1kb loader which returns
+control to the bootrom, after which it will load the real loader, which
+can then be up to 29kb in size and does the regular ddr init.
+
+Additionally the rk3188 requires everything the bootrom loads to be
+rc4-encrypted. Except for the very first stage the bootrom always reads
+and decodes 2kb pages, so files should be sized accordingly.
+
+# copy tpl, pad to 1020 bytes and append spl
+cat tpl/u-boot-tpl.bin > tplspl.bin
+truncate -s 1020 tplspl.bin
+cat spl/u-boot-spl.bin >> tplspl.bin
+tools/mkimage -n rk3188 -T rksd -d tplspl.bin out
+
+# truncate, encode and append u-boot.bin
+truncate -s %2048 u-boot.bin
+cat u-boot.bin | split -b 512 --filter='openssl rc4 -K 7C4E0304550509072D2C7B38170D1711' >> out
+
+
 Using fastboot on rk3288
 ========================
 - Write GPT partition layout to mmc device which fastboot want to use it to