plat-k3: drivers: Refactor SA2UL RNG firewall setup

sa2ul_init() contains code to set firewall for SA2UL RNG. However,
almost the same code can also be used to firewall DTHEv2 RNG. Therefore
refacto

plat-k3: drivers: Refactor SA2UL RNG firewall setup

sa2ul_init() contains code to set firewall for SA2UL RNG. However,
almost the same code can also be used to firewall DTHEv2 RNG. Therefore
refactor this code into a separate function in the ti_sci driver.

Signed-off-by: Suhaas Joshi <s-joshi@ti.com>
Reviewed-by: T Pratham <t-pratham@ti.com>
Reviewed-by: Andrew Davis <afd@ti.com>

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