phy: Break include cycle

Because some phy wants to export some functions [1], export.h was
including the whole phy subsystem which pulls in lots of stuff that
causes some ordering and redefinition issues. Split out the only part
that is actually needed in export.h and include it there and in phy.h.

[1] commit 95279315 ("board/ls2085rdb: Export functions for
			  standalone AQ FW load apps")

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>