spear: move CONFIG_SYS_I2C_BASE from arch-spear/hardware to board configs

Having CONFIG_SYS_I2C_BASE requires DW I2C driver to explicitly include
<arch/hardware.h> which other platforms may not have at all.

It's always good to have a driver platform-independent.

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>