drivers: phy: Add support for optional phys

drivers: phy: Add support for optional phys

Add devm_phy_optional_get and phy_optional_get, which should be used
when the phy is optional. They does not return an error when the phy
does not exist, rather they returns NULL, which is considered as a valid
phy, but results in NOPs when used with the consumer API.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>
[backport from upstream commit 788a4d56ff378bff0b8e685d03a962b36903a149]
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Change-Id: I76c9bb54d4bd11b742522c91c28f48b82ca4aa41