[ARM] 3064/1: start using ixp2000_reg_wrb

Patch from Lennert Buytenhek

Switch the users of ixp2000_reg_write that depend on writes being
flushed out of the write buffer by the time that function returns
over to ixp2000_reg_wrb.

When using XCB=101, writes to the same functional unit are still
guaranteed to complete in order, so we only need to protect against:
- reordering of writes to different functional units
- masking an interrupt and then reenabling the IRQ bit in CPSR

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Deepak Saxena <dsaxena@plexity.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>