ARM: dts: msm: add VDD_GFX retention corner for msm8996

Add a new lowest VDD_GFX CPR corner on msm8996 which corresponds
to the retention level of 400 mV.  Also update the GPU clock
frequency to corner mapping tables to account for the shifted
corner values.

Change-Id: If67f66b41d2fefafa0ba45addf9fbdf7aa3f81b2
Signed-off-by: David Collins <collinsd@codeaurora.org>