serial: 8250: Add cpufreq support

On DA850/OMAP-L138 SoC, the PLL which supplies the clock to CPU also
feeds the UART and the UART input frequency can change when the CPU
frequency is scaled.

This patch adds cpufreq support for 8250 serial driver. A clk structure
member has been added to the platform and port data structures.This
member is used by the cpufreq notifier callback to get the updated
clock rate. The implementation is based on the cpufreq implementation
for Samsung serial driver.

Tested on TI DA850/OMAP-L138 EVM.

Signed-off-by: Chaithrika U S <chaithrika@ti.com>
Signed-off-by: Shankarmurthy, Akshay <akshay.s@ti.com>