mx6: Fix calculation of emi_slow clock rate

mx6: Fix calculation of emi_slow clock rate

This is porting of Freescale's patch from version imx_v2009.08_3.0.35_4.0.0,
that fixes the obvious mistype of bits offset macro name (ACLK_EMI_PODF_OFFSET
was used instead of ACLK_EMI_SLOW_PODF_OFFSET).

Using the occasion, change the variable name 'emi_slow_pof' to more consistent
'emi_slow_podf'.

Signed-off-by: Jason Liu <r64343@freescale.com>
Signed-off-by: Andrew Gabbasov <andrew_gabbasov@mentor.com>
Acked-by: Dirk Behme <dirk.behme@de.bosch.com>