net: phy: realtek: make HWMON support a user-visible Kconfig symbol

Make config symbol REALTEK_PHY_HWMON user-visible, so that users can
remove support if not needed.

Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/3466ee92-166a-4b0f-9ae7-42b9e046f333@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>