ARM: BCM63xx: Remove unused pmb_dn variable

Introduced in commit 3f2a43c9 ("ARM: BCM63xx: Add secondary CPU PMB
initialization sequence"), but not used by the code.

Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>