net: phy: at803x: add missing dependency on CONFIG_REGULATOR

Compilation fails on PPC targets as CONFIG_REGULATOR is not set and
drivers/regulator/devres.c is not compiled in while functions exported
there are used by drivers/net/phy/at803x.c. Here's the error log:

  LD      .tmp_vmlinux1
drivers/net/phy/at803x.o: In function `at803x_rgmii_reg_set_voltage_sel':
drivers/net/phy/at803x.c:294: undefined reference to `.rdev_get_drvdata'
drivers/net/phy/at803x.o: In function `at803x_rgmii_reg_get_voltage_sel':
drivers/net/phy/at803x.c:306: undefined reference to `.rdev_get_drvdata'
drivers/net/phy/at803x.o: In function `at8031_register_regulators':
drivers/net/phy/at803x.c:359: undefined reference to `.devm_regulator_register'
drivers/net/phy/at803x.c:365: undefined reference to `.devm_regulator_register'
drivers/net/phy/at803x.o:(.data.rel+0x0): undefined reference to `regulator_list_voltage_table'
linux/Makefile:1074: recipe for target 'vmlinux' failed
make[1]: *** [vmlinux] Error 1

Fixes: 2f664823 ("net: phy: at803x: add device tree binding")
Signed-off-by: Madalin Bucur <madalin.bucur@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>