hw/armv7m_nvic: Implement byte/halfword access for NVIC SCB_SHPRx registers

hw/armv7m_nvic: Implement byte/halfword access for NVIC SCB_SHPRx registers

Implement byte/halfword read and write for the NVIC SCB_SHPRx
(System Handler Priority Registers).  Do this by removing SHPR word access
from nvic_readl/writel and adding common code to hande all access
sizes in nvic_sysreg_read/write.

Because the "nvic_state *s" variable now needs to be declared in
nvic_sysreg_read/write, the "void *opaque" parameter of
nvic_readl/writel is changed to "nvic_state *s".

Signed-off-by: Andre Beckus <mikemail98-qemu@yahoo.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>