lp8727_charger: Fix code for getting battery temp

For better understanding, use specific function and definitions rather
than magic numbers. New enum type for die temperature is matched with hex
codes. Specific temperature names are better than 0x1, 0x2 and 0x3.

And lp8727_is_high_temperature() function has better readability than
comparing raw register values.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>