drivers: net: Provide Kconfig menu for PHYLIB

Provide the necessary Kconfig symbols so that PHYLIB support may be
enabled in Kconfig, as opposed to needing to #define these symbols in
C source headers.

BITBANGMII and MV88E6352_SWITCH are left out of the PHYLIB submenu as
they don't seem to explicitly depend on it (i.e. they do not use the
phy_driver class).

Signed-off-by: Alexandru Gagniuc <alex.g@adaptrum.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>