riscv: Fix XIP_FIXUP_FLASH_OFFSET

There were several problems with the calculation. Not only was an 'and'
being computed into t1 but thrown away; but the 'and' itself would
cause problems if the granularity of the XIP physical address was less
than XIP_OFFSET - in my case I had the kernel image at 2MB in SPI flash.

Fixes: f9ace4ed

 ("riscv: remove .text section size limitation for XIP")
Cc: stable@vger.kernel.org
Signed-off-by: Myrtle Shah <gatecat@ds0.me>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>