boards: lilygo: ttgo_tbeam: initialize GNSS after regulator

As the initialization order is not yet defined by the device tree, we
manually need to reflect the order via the init priorities. As the GNSS
is connected to a UART that is powered by a regulator, the regulator
needs to be initialized first. By setting the prio to 87, we initialize
GNSS right after the regulator (instead of before as done without this
patch).

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