bcmasp: BCMASP should depend on ARCH_BRCMSTB

The Broadcom ASP 2.0 Ethernet controller is only present on Broadcom STB
SoCs.  Hence add a dependency on ARCH_BRCMSTB, to prevent asking the
user about this driver when configuring a kernel without Broadcom
ARM-based set-top box chipset support.

Fixes: 490cb412

 ("net: bcmasp: Add support for ASP2.0 Ethernet controller")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Acked-by: Florian Fainelli <florian.fainelli@broadcom.com>
Acked-by: Justin Chen <justin.chen@broadcom.com>
Link: https://lore.kernel.org/r/1e8b998aa8dcc6e38323e295ee2430b48245cc79.1690299794.git.geert+renesas@glider.be
Signed-off-by: Jakub Kicinski <kuba@kernel.org>