powerpc: mpc8xx: harmonise initialisation of the immap local pointer

powerpc: mpc8xx: harmonise initialisation of the immap local pointer

In most places, immap local pointer is defined as
	immap_t __iomem *immap = (immap_t __iomem *)CONFIG_SYS_IMMR;
In a few places, it is defined as
	immap_t __iomem *immap = (immap_t __iomem *)(immr & 0xFFFF0000);

This patch replaces the few of the latest form by the other one.

The two are fully equivalent since SPRN_IMMR is set with CONFIG_SYS_IMMR
very early in start.S

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>