hw/riscv: sifive_e: Fix the number of interrupt sources of PLIC

hw/riscv: sifive_e: Fix the number of interrupt sources of PLIC

Per chapter 10 in Freedom E310 manuals [1][2][3], E310 G002 and G003
supports 52 interrupt sources while G000 supports 51 interrupt sources.

We use the value of G002 and G003, so it is 53 (including source 0).

[1] G000 manual:
https://sifive.cdn.prismic.io/sifive/4faf3e34-4a42-4c2f-be9e-c77baa4928c7_fe310-g000-manual-v3p2.pdf

[2] G002 manual:
https://sifive.cdn.prismic.io/sifive/034760b5-ac6a-4b1c-911c-f4148bb2c4a5_fe310-g002-v1p5.pdf

[3] G003 manual:
https://sifive.cdn.prismic.io/sifive/3af39c59-6498-471e-9dab-5355a0d539eb_fe310-g003-manual.pdf

Fixes: eb637edb1241 ("SiFive Freedom E Series RISC-V Machine")
Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20221211030829.802437-11-bmeng@tinylab.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>