i2c: designware_i2c: Optionally check enable status register

Some platforms don't implement the enable status register at offset 0x9c.
The SPEAr600 platform is one of them. The recently added check to this
status register can't be performend on these platforms.

This patch introduces a new config option that can be enabled on such
platforms not supporting this register.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Reviewed-by: Heiko Schocher <hs@denx.de>