powerpc/p1_p2_rdb_pc: Fix warnings for __iomem pointers

Add the __iomem address space marker for the tsec pointers
to struct tsec_mii_mng memory mapped register regions.
This solves the sparse warnings for mixig normal pointers with
__iomem pointers for tsec.

p1_p2_rdb_pc.c:373:24: warning: incorrect type in assignment (different
address spaces)
p1_p2_rdb_pc.c:373:24:    expected struct tsec_mii_mng [noderef]
<asn:2>*regs
p1_p2_rdb_pc.c:373:24:    got struct tsec_mii_mng *<noident>

Use TSEC_GET_MDIO_REGS_BASE() for the remaining mdio 'regs'
initializations to remove the __iomem warnings and for consistency.

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Acked-by: York Sun <yorksun@freescale.com>