ENGR00229470-2 MX6SL-Add support for debug UART to be sourced from 24MHz.

ENGR00229470-2 MX6SL-Add support for debug UART to be sourced from 24MHz.

If "debug_uart" is specified in the command line, uart will
be sourced from 24MHz XTAL. This is required for getting the
correct power measurements on MX6SL.
Certain analog power optimizations are done only if ALL PLLs
are bypassed on MX6SL. To verify this path, we need to ensure
that UART is not sourced from PLL3.

Signed-off-by: Ranjani Vaidyanathan <ra5478@freescale.com>