net: phy: Kconfig: micrel_phy: fix dependency issue

When building driver CONFIG_MICREL_PHY the follow error shows up:

aarch64-linux-gnu-ld: drivers/net/phy/micrel.o: in function `lan8814_ts_info':
micrel.c:(.text+0x1764): undefined reference to `ptp_clock_index'
micrel.c:(.text+0x1764): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `ptp_clock_index'
aarch64-linux-gnu-ld: drivers/net/phy/micrel.o: in function `lan8814_probe':
micrel.c:(.text+0x4720): undefined reference to `ptp_clock_register'
micrel.c:(.text+0x4720): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `ptp_clock_register'

Rework Kconfig for MICREL_PHY to depend on 'PTP_1588_CLOCK_OPTIONAL'.
Arnd describes in a good way why its needed to add this depends in patch
e5f3155 ("ethernet: fix PTP_1588_CLOCK dependencies").

Reported-by: kernel test robot <lkp@intel.com>
Fixes: ece1950 ("net: phy: micrel: 1588 support for LAN8814 phy")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20220314110254.12498-1-anders.roxell@linaro.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/phy/Kconfig

@@ -220,6 +220,7 @@ config MEDIATEK_GE_PHY
 
 config MICREL_PHY
 	tristate "Micrel PHYs"
+	depends on PTP_1588_CLOCK_OPTIONAL
 	help
 	  Supports the KSZ9021, VSC8201, KS8001 PHYs.