clk: rockchip: rk3528: add slab.h header include

The newly added GRF types introduced kzalloc usage into the rk3528.
At least for the similar rk3576 driver, the kernel-test-robot reported the
missing prototype, which warranted adding a slab.h include.

While it did not complain about the rk3528, so the header might be included
"accidentially" right now, add a real include to make sure we keep it
included in the future.

Fixes: 306d2f5d ("clk: rockchip: rk3528: Add SD/SDIO tuning clocks in GRF region")
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20250515082652.2503063-2-heiko@sntech.de