boards: arm: mimxrt1015_evk: enable lpuart4 only if BT_UART is enabled

Since lpuart4 is enabled by default on mimxrt1015_evk but not configured
in the device tree, this leads to a build error after adding the
instance to the lpuart driver. Adjust the mimxrt1015_evk to be the same
as other mimxrt evk boards and enable the lpuart only if BT_UART is
enabled.

Signed-off-by: Filip Brozovic <fbrozovic@gmail.com>