stm32: common: fix missing SPI_I2S_BASE rename regression

Fix regresion from 5d4437fe43aa68a28611e9a0aecc712833230304 where
somehow not all occurrences were replaced.  I could swear it all
compiled when I was working on it :(

Reported-by: Trevor Woerner <trevor.woerner@linaro.org>
[f4] Added a 25mhz clock
Renamed README to README.md to be recognized as Markdown by GitHub
Markdowned README.md
[stm32f7] Add initial support of the family, GPIO support.

PENDING BUGS:

CHECK f7/irq.json No documentation for IRQ handlers yet released.
CHECK f7/Makefile No support for -mcpu=cortex-m7 Should be changed when GCC will support it
[stm32f7] Add doxygen support for F7 family