soc: bcm: brcmstb: add stubs for getting platform IDs

Some brcmstb drivers may be shared with other SoC families. E.g. the
same USB PHY block is shared by brcmstb and BCM4908.

To avoid building brcmstb common code on non-brcmstb platforms we need
stubs for:
1. brcmstb_get_family_id()
2. brcmstb_get_product_id()
(to avoid "undefined reference to" errors).

With this change PHY_BRCM_USB will not have to unconditionally select
SOC_BRCMSTB anymore.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>