irqchip/gic-v3-its: Rename VPENDBASER/VPROPBASER accessors

V{PEND,PROP}BASER registers are actually located in VLPI_base frame
of the *redistributor*. Rename their accessors to reflect this fact.

No functional changes.

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200206075711.1275-7-yuzenghui@huawei.com