riscv: Define PLATFORM__CLEAR_BIT for generic_clear_bit()

riscv bitops.h provides a __clear_bit() but does not define
PLATFORM__CLEAR_BIT as a result generic_clear_bit() is used instead of the
architecturally provided __clear_bit().

This patch defines PLATFORM__CLEAR_BIT which means that __clear_bit() in
riscv bitops.h will be called whenever generic_clear_bit() is called - as
opposed to the default cross-platform generic_clear_bit().

Signed-off-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Cc: Rick Chen <rick@andestech.com>
Cc: Greentime Hu <green.hu@gmail.com>