configs: am57xx_evm: increase PHY autonegotiate time

Increase PHY autonegoiate time from current 8 seconds
to 16 seconds. On some Ethernet switches, 2 times out
of 100, the Micrel KSZ9031 PHY on AM572x IDK board seems
to take more than 8 seconds to establish link at gigabit
speeds.

Since the timeout is only an upper bound on waiting time
it should not affect users who do not face the same
problem.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>