add operating definitions of UART0 to UART5
support UART0 to UART5 straightly
add RT_USING_UARTx and set console via uart0
add CM_PER_UARTx_CLKCTRL
friendly for uart count under 6
Merge pull request #227 from RTsien/master

Add supports for UART0 to UART5 of beaglebone(black)