riscv: fix RISCV_ISA_SVPBMT kconfig dependency warning

RISCV_ISA_SVPBMT selects RISCV_ALTERNATIVE which depends on !XIP_KERNEL.
Therefore RISCV_ISA_SVPBMT should also depend on !XIP_KERNEL so
quieten this kconfig warning:

WARNING: unmet direct dependencies detected for RISCV_ALTERNATIVE
  Depends on [n]: !XIP_KERNEL [=y]
  Selected by [y]:
  - RISCV_ISA_SVPBMT [=y] && 64BIT [=y] && MMU [=y]

Fixes: ff689fd2 ("riscv: add RISC-V Svpbmt extension support")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: stable@vger.kernel.org
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20220709014929.14221-1-rdunlap@infradead.org/


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