[dif/sysrst_ctrl] add initial header
[dif/sysrst_ctrl] add initial header

This adds an initial header for the System Reset Controller DIF library.
Included in this initial API are DIFs to configure and operate:

1. the key combo detection feature,
2. the key input signal change detection feature, and
3. the key auto-override feature

Signed-off-by: Timothy Trippel <77eacc84f9735d3cbb55a271710be269fc3a5818@google.com>

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