net: cavium: Fix build errors due to 'imply CAVIUM_PTP'

If CAVIUM_PTP is m and THUNDER_NIC_VF is y, build fails:

drivers/net/ethernet/cavium/thunder/nicvf_main.o: In function 'nicvf_remove':
nicvf_main.c:(.text+0x1f0): undefined reference to 'cavium_ptp_put'
drivers/net/ethernet/cavium/thunder/nicvf_main.o: In function `nicvf_probe':
nicvf_main.c:(.text+0x557c): undefined reference to 'cavium_ptp_get'

THUNDER_NIC_VF imply CAVIUM_PTP, which allow the config now,
Use IS_REACHABLE() to avoid the vmlinux link error for this case.

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: def2fbff

 ("kconfig: allow symbols implied by y to become m")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>