doc: imx: habv4: Add HABv4 introduction

doc: imx: habv4: Add HABv4 introduction

The HABv4 is supported in i.MX50, i.MX53, i.MX6, i.MX7,
series and i.MX 8M, i.MX8MM devices.

Add an introductory document containing the following topics:

- HABv4 Introduction
- HABv4 Secure Boot
- HABv4 Encrypted Boot
- HAB PKI tree generation
- HAB Fast Authentication PKI tree generation
- SRK Table and SRK Hash generation

Reviewed-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Utkarsh Gupta <utkarsh.gupta@nxp.com>
Signed-off-by: Breno Lima <breno.lima@nxp.com>