leds: qpnp-flash-v2: Update the flash_prepare API

Update the flash prepare API to accept a led_trigger device as an
argument. Also split the leds-qpnp-flash-v2 header file to keep the
flash_prepare API in a common header file for both legacy and v2
flash drivers.

CRs-Fixed: 1043718
Change-Id: Idb4ab086740b680e101d76d882bcfb618ac4b936
Signed-off-by: Devesh Jhunjhunwala <deveshj@codeaurora.org>