ARM: OMAP4+: Move external phy initialisations to arch specific place.

The external phy is present in the case OMAP5 soc is currently
configured in emif-common.c. This results in having dummy structures
for those Socs which do not have a external phy. So by having a weak
function in emif-common and overriding it in OMAP5, avoids the use
of dummy structures.

Signed-off-by: R Sricharan <r.sricharan@ti.com>