soc: bcm: brcmstb: depend on ARCH_BRCMSTB over arm arches

The SOC_BRCMSTB is better depending on ARCH_BRCMSTB as it's
defined for both arm and aarch64 but is more specific to
the actual SoC's dependencies, it'll still get compile tested.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Link: https://lore.kernel.org/r/20231009135833.17880-2-pbrobinson@gmail.com


Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>