regulator: qpnp-lcdb: Fix boost headroom and max voltage configuration

For PM660L, maximum boost voltage is 6250 mV and for PM855L, it
is 6275 mV. Fix this. Also, if the child node for boost regulator
is not specified in the device tree, boost headroom is set to 0.
This is incorrect when there is a request for voltage change in
addition to enable/disable LDO/NCP regulators, boost voltage can
get set without the required headroom as the headroom was set to
0. Fix this by initializing boost headroom to default values even
if the child node for boost is not defined.

While at it, add the default boost headroom values for different
PMICs in device tree bindings documentation.

Change-Id: I79911d88e0fa25d8b520796c8eea61e6e4ab27ae
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>