venus: firmware: Correct assertion of reset bit on remote processor

Currently we use read/write_relaxed in combination with mb() to assert
reset. This looks wrong because mb() after write_relaxed() will not
order correctly load-update-store sequence. Correct this by use
readl/writel which include memory barriers.

Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>