powerpc/85xx: Enhance get_sys_info() to check clocking mode

T1040 and it's variants provide "Single Oscillator Source" Reference Clock Mode.

In this mode, single onboard oscillator(DIFF_SYSCLK) can provide the reference clock
(100MHz) to the following PLLs:
• Platform PLL
• Core PLLs
• USB PLL
• DDR PLL, etc

The cfg_eng_use0 of porsr1 register identifies whether the SYSCLK (single-ended) or
DIFF_SYSCLK (differential) is selected as the clock input to the chip.

get_sys_info has been enhanced to add the diff_sysclk so that the
various drivers can be made aware of ths diff sysclk configuration and
act accordingly.

Other changes:
-single_src to ddr_refclk_sel, as it is use for checking ddr reference clock
-Removed the print of single_src from get_sys_info as this will be
-printed whenever somebody calls get_sys_info which is not appropriate.
-Add print of single_src in checkcpu as it is called only once during initialization

Signed-off-by: Poonam Aggrwal <poonam.aggrwal@freescale.com>
Signed-off-by: Priyanka Jain <Priyanka.Jain@freescale.com>
Signed-off-by: Vijay Rai <vijay.rai@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>