arc: remove "hardware.h"

This file was only required for compilation of designware_i2c driver.
Since explicit inclusion of "hardware.h" is now removed from the driver
we may safely remove this empty header as well.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>

Cc: Tom Rini <trini@ti.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Vipin Kumar <vipin.kumar@st.com>
Cc: Armando Visconti <armando.visconti@st.com>