TM4C1294KCPDT: CAS latency

Hi Morita-san,

  The CAS latency is fixed at 2 for TM4C129. We have tested the TM4C129 parts with MT48LC32M16A2TG-75 which supports the CAS Latency of 0x2

11.5.2 SDRAM Mode
When activating the SDRAM mode, it is important to consider a few points:
1. Generally, it takes over 100 μs from when the mode is activated to when the first operation is
allowed. The SDRAM controller begins the SDRAM initialization sequence as soon as the mode
is selected and enabled via the EPICFG register. It is important that the GPIOs are properly
configured before the SDRAM mode is enabled, as the EPI controller is relying on the GPIO
block's ability to drive the pins immediately. As part of the initialization sequence, the LOAD
MODE REGISTER command is automatically sent to the SDRAM with a value of 0x27, which
sets a CAS latency of 2 and a full page burst length.