Replace russian comments in AT91SAM7A3 startup to english.

Fix overflow error in clock(): no reset of overflow 12-bit counter, so
max value of clock() was 2^12 - 1 = 4095 ms. New code reset overflow
counter and user must call clock() with period less that 4095 ms.

Adding some documentaion and LICENSE to AT91SAM7A3 port.
Merge pull request #430 from bortsov/armcc

Replace russian comments in AT91SAM7A3 startup to english.