ASoC: fsl_ssi: use flat regmap cache

Same as commit ce492b3b
Subject: drm/fsl-dcu: use flat regmap cache

Using flat regmap cache instead of RB-tree to avoid the following
lockdep warning on driver load:
WARNING: CPU: 0 PID: 1 at kernel/locking/lockdep.c:2871 lockdep_trace_alloc+0x104/0x128
DEBUG_LOCKS_WARN_ON(irqs_disabled_flags(flags))

The RB-tree regmap cache needs to allocate new space on first
writes. However, allocations in an atomic context (e.g. when a
spinlock is held) are not allowed. The function regmap_write
calls map->lock, which acquires a spinlock in the fast_io case.
Since the driver uses MMIO, the regmap bus of type regmap_mmio
is being used which has fast_io set to true.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>