[L] Change in coreboot[master]: mb/google/rex: Enable ISH support

[Bernardo Perez Priego (Code Review)]

Attention is currently required from: Boris Mittelberg, Caveh Jalali, Gwendal Grignou, Harsha B R, Jamie Ryu, Jay Patel, Kapil Porwal, Li1 Feng, Paul Menzel, Subrata Banik, Tarun Tuli, Vijay P Hiremath, Wonkyu Kim.

Bernardo Perez Priego has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/75039?usp=email )

Change subject: mb/google/rex: Enable ISH support
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Patch Set 13:

(1 comment)

File src/mainboard/google/rex/Kconfig:

https://review.coreboot.org/c/coreboot/+/75039/comment/a10aef5b_92846313 :
PS13, Line 111: 	default "variants/$(CONFIG_VARIANT_DIR)/overridetree_ec_ish.cb" if DRIVERS_INTEL_ISH
> why is this required ? […]

I would think so. Including the ISH into devicetree requires `DRIVERS_INTEL_ISH`, which is only selected by `BOARD_GOOGLE_REX_EC_ISH`. See [75470/Kconfig.name](https://review.coreboot.org/c/coreboot/+/75470/2/src/mainboard/google/rex/Kc...)

The change below in overridetree.cb without `DRIVERS_INTEL_ISH` causes build failure.
```
    device ref ish on
    	probe ISH ISH_ENABLE
    	chip drivers/intel/ish
    		register "firmware_name" = ""rex_ish.bin""
    		device generic 0 on end
    	end
    end
```