To avoid hang-up of the Ethernet block after Cortex-M3 software reset
(SYSRESET), we need to reset the Ethernet PHY immediately before
performing the SYSRESET.

All new code added in this patch should be in placed in `.ramcode`,
because we might want to do a software reset after self-upgrade.

Since we cannot use `printf()` in functions that may be called during
self-upgrade (`printf()` is too big for `.ramcode`), the
`lpc178x_phy_init()` function cannot be easily used in
`lpc178x_phy_final_reset()`. Because of this, we use a pre-set PHY
address (`CONFIG_LPC178X_ETH_PHY_ADDR`) instead of doing automatic
PHY discovery that is usually done in `lpc178x_phy_init()`.

If Ethernet is not enabled in the U-Boot configuration file, we do not
perform the PHY reset.
This leads to a minor bug: if you install U-Boot without Ethernet
support into your board and do a self-upgrade to another build of
U-Boot with Ethernet support, the Ethernet driver will hang in the
latter U-Boot unless you have done a full reset (by pushing the SW1
button) after self-upgrade.