target/openrisc: Fix memory reading in debugger

target/openrisc: Fix memory reading in debugger

In commit f0655423ca ("target/openrisc: Reorg tlb lookup") data and
instruction TLB reads were combined.  This, broke debugger reads where
we first tried to map using the data tlb then fall back to the
instruction tlb.

This patch replicates this logic by first requesting a PAGE_READ
protection mapping then falling back to PAGE_EXEC.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Stafford Horne <shorne@gmail.com>