lm4f: Add system control register definitions

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
lm4f: Update stellaris example to new LM4F registers

The LM4F provides new registers for system control. Each peripheral
now has its own clock gating control register. The LM4F still supports
the legacy registers for backwards compatibility.

Use SYSCTL_RCGCGPIO instead of SYSCTL_RCGC to enable the GPIO.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
doc: Include LM4F files when creating documentation

There was no reference to LM4F in in the doc/ subdirectory. Add the needed
infrastructure to generate focumentation for LM4F.

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