Add global variable sdhc_per_clk for arm/powerpc

The QorIQ eSDHC controller supports two reference clocks. They are
platform clock and periperhal clock. The global variable sdhc_clk
has already been used for platform clock.
This patch is to add another global variable sdhc_per_clk for
periperhal clock, which provides higher frequency and is required
to be used for SD UHS and eMMC HS200/HS400 speed modes.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>