bfin_mac: Restore hardware time-stamping dependency on BF518

Commit 70ac618c ("ptp: fixup Kconfig for two PHC drivers.") removed all
dependencies for the blackfin hardware time-stamping Kconfig entry. Hardware
time-stamping is only available on BF518 though. Since the Kconfig entry is
'default y', just updateing your kernel source and running `make defconfig` will
result in the the following build errors:

	drivers/net/ethernet/adi/bfin_mac.c:694: error: implicit declaration of function ‘bfin_read_EMAC_PTP_CTL’
	drivers/net/ethernet/adi/bfin_mac.c:702: error: implicit declaration of function ‘bfin_write_EMAC_PTP_FV3’
	drivers/net/ethernet/adi/bfin_mac.c:712: error: implicit declaration of function ‘bfin_write_EMAC_PTP_CTL’
	drivers/net/ethernet/adi/bfin_mac.c:717: error: implicit declaration of function ‘bfin_write_EMAC_PTP_FOFF’
	...

This patch adds back the dependency on BF518, and since it does not make sense
to expose this config option when the blackfin MAC driver is not enabled also
restore the dependency on BFIN_MAC.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>