kernel.bbclass: State riscv required tune_features for Linux

kernel.bbclass: State riscv required tune_features for Linux

Required:
   rv32ima_zicsr_zifencei
   rv64ima_zicsr_zifencei

See the arch/riscv/Makefile:

riscv-march-$(CONFIG_ARCH_RV32I)	:= rv32ima
riscv-march-$(CONFIG_ARCH_RV64I)	:= rv64ima
riscv-march-$(CONFIG_TOOLCHAIN_NEEDS_EXPLICIT_ZICSR_ZIFENCEI) := $(riscv-march-y)_zicsr_zifencei

Signed-off-by: Mark Hatle <mark.hatle@amd.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>