Documentation: add Intel Broadwell

Change-Id: I0b1c29162a64030b5c100368f2471702e22b8311
Signed-off-by: Iru Cai <mytbk920423@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/54744
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

Documentation/soc/intel/broadwell/index.md

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+# Intel Broadwell documentation
+
+This section describes the Intel Broadwell SoC.
+
+## Proprietary blobs
+
+- [mrc.bin and refcode](blobs.md)