clk: si521xx: Use REGCACHE_FLAT instead of NONE

In order to reload registers into the clock generator on resume using
regcache_sync(), it is necessary to select one of the regcache types
which are not NONE. Since this device has some 7 registers, use the
simplest one, FLAT. The regcache code complains about REGCACHE_NONE
being selected and generates a WARNING, this fixes that warning.

Fixes: edc12763 ("clk: si521xx: Clock driver for Skyworks Si521xx I2C PCIe clock generators")
Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20230831181656.154750-1-marex@denx.deSigned-off-by: Stephen Boyd <sboyd@kernel.org>

drivers/clk/clk-si521xx.c

@@ -146,7 +146,7 @@ static int si521xx_regmap_i2c_read(void *context, unsigned int reg,
 static const struct regmap_config si521xx_regmap_config = {
 	.reg_bits = 8,
 	.val_bits = 8,
-	.cache_type = REGCACHE_NONE,
+	.cache_type = REGCACHE_FLAT,
 	.max_register = SI521XX_REG_DA,
 	.rd_table = &si521xx_readable_table,
 	.wr_table = &si521xx_writeable_table,