ARM: 8727/1: MAINTAINERS: Update brcmstb entries to cover B15 code

Update the brcmstb entry to cover the Broadcom Brahma-B15 processor
read-ahead cache support code.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>