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.de
Signed-off-by: Stephen Boyd <sboyd@kernel.org>