phy: pistachio-usb: Depend on MIPS || COMPILE_TEST

So it will be avilable for generic MIPS kernel.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Acked-By: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>