What this NPN transistor circuit do?
The only reference to "MFB" in the CSR64215 datasheet is the VREGENABLE signal on pin 40. In the datasheet there's a pushbutton switch to connecting VREGENABLE to Vbat.

The circuit you show is similar to the VREGENABLE delay circuit on page 69 of the datasheet:



How does this circuit work?

When the supply voltage (1V8_INPUT, VBAT in your circuit) becomes active the capacitor will charge. This causes a current to flow through the resistor in series with the capacitor (R7 in your schematic). This then activates the NMOS or NPN which will then pull down the output VREGENABLE (MFB in your circuit).

After some time (about RC = 10 kohm * 1 uF = 10 ms, this is a ballpark figure, not very accurate!), a couple of milli seconds, the capacitor is charged and the charging current becomes zero. Then there is no voltage to activate the NMOS / NPN anymore so it opens and the VREGENABLE output (MFB in your circuit) becomes equal to the supply voltage (1V8_INPUT, VBAT in your circuit).

So your circuit makes MFB high after some delay, when VBAT is switched on.