Designing a gate voltage for N-channel MOSFETS for H-bridge controller
5V is barely enough to turn on an IRF540. The specified 0.077 Ohms Rdson is achieved with 10V Gate drive. You are probably getting away with the lower drive voltage because your motors are only drawing a small current.  

The simplest way to get a voltage higher than the supply is to make a 'charge pump' using a couple of capacitors and diodes. You also need a driving waveform with sufficient voltage and current, which could be produced by a 555 timer, CMOS gates, an op-amp etc. Here's an example circuit:-







simulate this circuit &ndash; Schematic created using CircuitLab

This circuit should produce about 22V (10V of boost) at 5mA. Note that this voltage exceeds the FETs Gate voltage rating, so you must limit the Gate-Source voltage to less than 20V.