pwm: qti-lpg: Support two-nvmem PPG scheme

On PMICs such as PM8350C, the lookup table containing the pattern data
is stored in a separate nvmem device from the one where the per-channel
data is stored.

Add two separate nvmems to handle this case while maintaining backward
compatibility for those targets that use only a single nvmem device.
Differentiate between the two schemes via an enum.

Change-Id: I2826a9acc8cee4d83b6cf2bf2d4d60b1ec3b67d7
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>