clk: sifive: allocate sufficient memory for struct __prci_data

The (struct __prci_data).hw_clks.hws is an array with dynamic elements.
Using struct_size(pd, hw_clks.hws, ARRAY_SIZE(__prci_init_clocks))
instead of sizeof(*pd) to get the correct memory size of
struct __prci_data for sifive/fu540-prci. After applying this
modifications, the kernel runs smoothly with CONFIG_SLAB_FREELIST_RANDOM
enabled on the HiFive unleashed board.

Fixes: 30b8e27e

 ("clk: sifive: add a driver for the SiFive FU540 PRCI IP block")
Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>