phy: core: document phy_calibrate()

Commit 36914111 ("drivers: phy: add calibrate method") added support
for generic phy_calibrate() method, but it didn't explain in detail when
such method is supposed to be called. Add some more documentation directly
to the phy.h to make it clean that it is intended to be called after every
host controller reset.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>