uboot-imx

stm32f4: add cpu clock option for 180 MHz

While most stm32f4 run at 168 MHz, stm32f429 can work till 180 MHz.
Add option to select 180 MHz through macro CONFIG_SYS_CLK_FREQ.

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
To: Albert Aribaud <albert.u.boot@aribaud.net>
To: Tom Rini <trini@konsulko.com>
To: Kamil Lulko <rev13@wp.pl>
Cc: u-boot@lists.denx.de