riscv: fix dependency for t-head errata

alternatives only work correctly on non-xip-kernels and while the
selected alternative-symbol has the correct dependency the symbol
selecting it also needs that dependency.

So add the missing dependency to the T-Head errata Kconfig symbol.

Reported-by: kernel test robot <yujie.liu@intel.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20220526205646.258337-5-heiko@sntech.de
Fixes: a35707c ("riscv: add memory-type errata for T-Head")
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/Kconfig.erratas

@@ -35,6 +35,7 @@ config ERRATA_SIFIVE_CIP_1200
 
 config ERRATA_THEAD
 	bool "T-HEAD errata"
+	depends on !XIP_KERNEL
 	select RISCV_ALTERNATIVE
 	help
 	  All T-HEAD errata Kconfig depend on this Kconfig. Disabling