pwm: qpnp: support DTEST configuration for PWM subtype

Currently, DTEST configuration is supported only based on the
DTEST line and output values for LPG subtype. Though this will
help configuring DTEST mode for PWM subtype, input validation
has to be fixed for supporting the latter properly. Add support
for that.

Also, rename the "lpg-dtest-line" device tree property to
"dtest-line" as this will apply for both LPG and PWM subtypes.

CRs-Fixed: 949595
Change-Id: I96bf477a14bb135cf9196532cf4bf39a45c9ff77
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>