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: a35707c3

 ("riscv: add memory-type errata for T-Head")
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>