leds: qpnp-flash: Introduce a "qpnp_flash_register_led_prepare"

Currently legacy "qpnp-flash-v2" and "qti-flash" drivers exposes an API
which needs to be called by the client drivers. The name of the APIs
are different in both flash drivers and both the drivers need to be
enabled in a single defconfig. To keep the client interface un-changed
introduce a function to which the individual flash drivers can register
their exposed APIs. Using this mechanism clients can use a common API
irrespective of the flash driver in use for that specific hardware.

Change-Id: I1ded3d4d9013c6f7f56e0a8952f4b45d20b07326
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>