Have PCIE_BRCMSTB depend on ARCH_BRCMSTB.  Also set the default value to
ARCH_BRCMSTB.

Link: https://lore.kernel.org/r/20200911175232.19016-2-james.quinlan@broadcom.comSigned-off-by: Jim Quinlan <jquinlan@broadcom.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>