clk: sunxi: Do not select the PRCM MFD

The PRCM MFD driver is already selected by the two platforms where it is
actually used (MACH_SUN6I and MACH_SUN8I). Selecting it here builds it
unnecessarily on the rest of the Allwinner platforms.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20220702190135.51744-2-samuel@sholland.org