pwm: pwm-qti-lpg: Refactor qpnp_lpg_parse_dt() for readability

Add helper functions to parse pattern-related properties and related
init logic, and clarify the qpnp_lpg_parse_dt function to make the code
flow clear in the cases wherein either SDAM or LUT, or neither of the
two is specified. Also add of_node_put() statements for the PBS node in
error paths.

Change-Id: I8055c61db9a8283e6c4d9dacf57012ff1d9227dd
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>