riscv: andes_plic: Fix some wrong configurations

Fix two wrong settings of andes plic driver as below:

1. Fix wrong pending register base definition.
2. Declaring the en variable in enable_ipi() as unsigned int instead of
   int can help to fix wrong plic enabling setting in RV64.

Signed-off-by: Rick Chen <rick@andestech.com>
Cc: KC Lin <kclin@andestech.com>
Cc: Alan Kao <alankao@andestech.com>