MAINTAINERS: rectify BROADCOM PMB (POWER MANAGEMENT BUS) DRIVER

Commit 8bcac401

 ("soc: bcm: add PM driver for Broadcom's PMB") includes
a new MAINTAINERS section BROADCOM PMB (POWER MANAGEMENT BUS) DRIVER with
'drivers/soc/bcm/bcm-pmb.c', but the file was actually added at
'drivers/soc/bcm/bcm63xx/bcm-pmb.c'.

Hence, ./scripts/get_maintainer.pl --self-test=patterns complains:

  warning: no file matches  F:    drivers/soc/bcm/bcm-pmb.c

Point the file entry to the right location.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>