powerpc: fix regression in arch_initr_trap()

powerpc: fix regression in arch_initr_trap()

The assembly output of the arch_initr_trap() function differed by a
single byte after common.h was removed from traps.c:

 fff49a18 <arch_initr_trap>:
 fff49a18:      94 21 ff f0     stwu    r1,-16(r1)
 fff49a1c:      7c 08 02 a6     mflr    r0
 fff49a20:      90 01 00 14     stw     r0,20(r1)
-fff49a24:      80 62 00 44     lwz     r3,68(r2)
+fff49a24:      80 62 00 38     lwz     r3,56(r2)
 fff49a28:      4b ff 76 19     bl      fff41040 <trap_init>
 fff49a2c:      80 01 00 14     lwz     r0,20(r1)
 fff49a30:      38 60 00 00     li      r3,0
 fff49a34:      38 21 00 10     addi    r1,r1,16
 fff49a38:      7c 08 03 a6     mtlr    r0

This was causing a consistent hard lockup during the MMC read / loading
of the QoriQ FMan firmware on a P2041RDB board.

Re-adding the header causes identical assembly to be emitted and allows
the firmware loading and subsequent boot to succeed.

Fixes: 401d1c4f5d ("common: Drop asm/global_data.h from common header")
Signed-off-by: Matt Merhar <mattmerhar@protonmail.com>