debugcon: make debug message more readable

debugcon: make debug message more readable

before change:
Bdebugcon: write addr=0x0000 val=0x6f
odebugcon: write addr=0x0000 val=0x6f
odebugcon: write addr=0x0000 val=0x74
tdebugcon: write addr=0x0000 val=0x69
idebugcon: write addr=0x0000 val=0x6e
ndebugcon: write addr=0x0000 val=0x67
gdebugcon: write addr=0x0000 val=0x20
 debugcon: write addr=0x0000 val=0x66

after change:
B [debugcon: write addr=0x0000 val=0x6f]
o [debugcon: write addr=0x0000 val=0x6f]
o [debugcon: write addr=0x0000 val=0x74]
t [debugcon: write addr=0x0000 val=0x69]
i [debugcon: write addr=0x0000 val=0x6e]
n [debugcon: write addr=0x0000 val=0x67]
g [debugcon: write addr=0x0000 val=0x20]
  [debugcon: write addr=0x0000 val=0x66]

Signed-off-by: liguang <lig.fnst@cn.fujitsu.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>