microblaze: cache: improve dcache Kconfig options

microblaze: cache: improve dcache Kconfig options

Replace CONFIG_DCACHE with a Kconfig option more limited in scope -
XILINX_MICROBLAZE0_USE_WDC. It should be enabled if the processor supports
the "wdc" (Write to Data Cache) instruction. It will be used to guard
"wdc" invocations in microblaze cache code.

Also, drop all ifdefs around flush_cache() calls and only keep one
CONFIG_IS_ENABLED() guard within flush_cache() itself.

Signed-off-by: Ovidiu Panait <ovpanait@gmail.com>
Link: https://lore.kernel.org/r/20220531181435.3473549-5-ovpanait@gmail.com
Signed-off-by: Michal Simek <michal.simek@amd.com>