NetsecDxe: put phy in loopback mode to guarantee stable RXCLK input

NETSEC hardware requires stable RXCLK input upon initialization
triggered with DISCORE = 0.
However, RXCLK input could be unstable depending on phy chipset
and deployed network environment, which could cause NETSEC to
hang up during initialization.

We solve this platform/environment dependent issue by temporarily
putting phy in loopback mode, then we can expect the stable RXCLK input.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>