rtl8195am - fix us_ticker interrupt handling
rtl8195am - fix us_ticker interrupt handling

TIMER2_7_IRQ is shared among several timer sources, including
us_ticker. Raising TIMER2_7_IRQ pending bit will trigger the timer
interrupt, but the timer interrupt handler will not know which timer
source this interrupt is for. This patch sets timer load value to one
tick and force us_ticker to fire almost "immediately".

TIMER2_7_IRQ is handled through a common interrupt handler, and
is automatically cleared. Therefore, there is no need to clear IRQ.

The underlying timer HAL treats load value as micro-seconds and does
conversion internally. Therefore, simply pass micro-seconds to timer
HAL without converting to tick first.

Signed-off-by: Tony Wu <38d699a2ec8283e63ba2d5893eaaac3c6cd66428@gmail.com>

ryankurte/mbed-os,andcor02/mbed-os,infinnovation/mbed-os,betzw/mbed-os,mbedmicro/mbed,kjbracey-arm/mbed,ryankurte/mbed-os,andcor02/mbed-os,CalSol/mbed,infinnovation/mbed-os,karsev/mbed-os,c1728p9/mbed-os,mazimkhan/mbed-os,CalSol/mbed,mbedmicro/mbed,infinnovation/mbed-os,karsev/mbed-os,karsev/mbed-os,mbedmicro/mbed,andcor02/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed,c1728p9/mbed-os,CalSol/mbed,mazimkhan/mbed-os,Archcady/mbed-os,betzw/mbed-os,ryankurte/mbed-os,c1728p9/mbed-os,Archcady/mbed-os,betzw/mbed-os,infinnovation/mbed-os,betzw/mbed-os,CalSol/mbed,CalSol/mbed,c1728p9/mbed-os,mazimkhan/mbed-os,c1728p9/mbed-os,ryankurte/mbed-os,Archcady/mbed-os,infinnovation/mbed-os,Archcady/mbed-os,Archcady/mbed-os,ryankurte/mbed-os,mazimkhan/mbed-os,CalSol/mbed,mazimkhan/mbed-os,mbedmicro/mbed,Archcady/mbed-os,karsev/mbed-os,betzw/mbed-os,andcor02/mbed-os,karsev/mbed-os,andcor02/mbed-os,mbedmicro/mbed,mazimkhan/mbed-os,c1728p9/mbed-os,ryankurte/mbed-os,betzw/mbed-os,andcor02/mbed-os,karsev/mbed-os,infinnovation/mbed-os,kjbracey-arm/mbed