In CPU definitions we should not be explicilty naming peripheral modules to use
via USEMODULE (one should use FEATURES_PROVIDED instead).
Plus add missing cpu_init() and periph_init() methods.
This commit removes periph_hwrng support from the pic32mx cpu builds as
pic32mx does not have hwrng only pic32mz does.
U3RXREG is the RX buffer for UART3. U3RXR is the register for
controlling the input pin for UART3.
Signed-off-by: Francois Berder <francois.berder@imgtec.com>
