dts: lilygo/ttgo_tbeam: add axp2101 charger node

The board provides an axp2101 chip with a builtin charger. We add the
corresponding charger node with values taken from the official XPowers
library example. This makes the board compatible with the
drivers/charger example.

[1] https://github.com/lewisxhe/XPowersLib/blob/a7d06b98c1136c8fee7854b1d29a9f012b2aba83/examples/AXP2101_Charge_Example/AXP2101_Charge_Example.ino



Signed-off-by: Felix Moessbauer <felix.moessbauer@gmail.com>