RISC-V: Factor out body of riscv_init_cbom_blocksize loop

Refactor riscv_init_cbom_blocksize() to prepare for it to be used
for both cbom block size and cboz block size.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230224162631.405473-3-ajones@ventanamicro.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>