[stm32] usart6 for F2/F4
[stm32] update doxy for usart_set_baudrate
lm4f: Properly set PLL divisor

rcc_set_pll_divisor() would take the number we wanted to divide the 400MHz
clock and put it directly in the RCC2 register. This caused the clock to always
be one speed tier slower than expected. The value of the divisor must be
decremented by 1, so a divisor of 5 will be written as 4 in the RCC2.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
lm4f: Fix include guard in nvic.h

The lm4f/nvic.h include guard was wrongfully named LIBOPENCM3_LM3S_NVIC_H.
This caused the lm3s/nvic.h include a few lines down to not be compiled.
Rename the inlcude guard to the more appropriate LIBOPENCM3_LM4F_NVIC_H.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
sam3x support.
sam3x: clean up gpio mess.
sam3x: Add some PMC convenience functions.
sam3x: Add USART registers and come convenience functions.
sam3x: Add gpio_init convenience function.
sam3x: Add some pwm convenience funcitons.
sam3x: Correct pmc_xtal_enable to use startup time parameter.
sam3x: Add missing return types to pwm functions.
sam3x: Add convenince functions for setting clocks and usart baudrate.
sam3x: Add usart funciton to enable rx interrupt.
Minor fix to compile functions in rtc.h
lm4f-doc: Sanitize doxygen structure in rcc.h

By somo copypaste error, the doxygen group rcc_defines was defined twice.
Don't do that. Keep Ken's doxygen structure and drop the old one.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Added *.tag files and the new generated sam3x include subdirectory to gitignores.
lm4f: Add UART register definitions

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
lm4f: Add UART "convenience" macros

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
lm4f: Add basic UART implementation

Add basic functionality needed to setup the UART and send/recieve data.
Interrupts, DMA, and more advanced features are not implemented in this patch.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>