platform: Add platform_find_device_by_driver() helper

Provide a helper to lookup platform devices by matching device
driver in order to avoid drivers trying to use platform bus
internals.

Cc: Eric Anholt <eric@anholt.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Heiko Stübner" <heiko@sntech.de>
Cc: Inki Dae <inki.dae@samsung.com>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Sandy Huang <hjc@rock-chips.com>
Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20190723221838.12024-8-suzuki.poulose@arm.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>