net: broadcom: Drop OF dependency from BGMAC_PLATFORM

All of the OF code that is used has stubbed and will compile and link
just fine, keeping COMPILE_TEST is enough.

Signed-off-by: Florian Fainelli <[e-mail]>
Acked-by: Randy Dunlap <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ethernet/broadcom/Kconfig

···
 config BGMAC_PLATFORM
 	tristate "Broadcom iProc GBit platform support"
 	depends on ARCH_BCM_IPROC || COMPILE_TEST
-	depends on OF
 	select BGMAC
 	select PHYLIB
 	select FIXED_PHY