[ARM] 2940/1: Fix BTB entry flush in arch/arm/mm/cache-v6.S

Patch from Gen FUKATSU

Invalidate BTB entry instruction flushes two instruction
at a time. Therefore this instruction should be done four
times after invalidate instruction cache line.

Signed-off-by: Gen Fukatsu
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>