phy: phy-brcm-usb: select SOC_BRCMSTB on brcmstb only

phy-brcm-usb has some conditional init code required on selected brcmstb
devices. Execution of that code depends on family / product detected by
brcmstb soc code.

For ARCH_BCM4908 brcmstb soc code always return 0 values as ids. Don't
bother selecting & compiling that redundant driver.

Depends-on: 149ae80b ("soc: bcm: brcmstb: add stubs for getting platform IDs")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20210305152406.2588-1-zajec5@gmail.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/phy/broadcom/Kconfig

@@ -94,7 +94,7 @@ config PHY_BRCM_USB
 	depends on ARCH_BCM4908 || ARCH_BRCMSTB || COMPILE_TEST
 	depends on OF
 	select GENERIC_PHY
-	select SOC_BRCMSTB
+	select SOC_BRCMSTB if ARCH_BRCMSTB
 	default ARCH_BCM4908
 	default ARCH_BRCMSTB
 	help